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4x5 Ultra Fine Focusing and Calibration
For Sale:
Loupe and Stands for 4x5 Precision Plenum Photography
Focusing with GG (Ground Glass) is crude and inaccurate.
Offered here is a shimmed plenum and loupe that allows you to verify
that your emulsion plane and focal plane are aligned within .001"
by bringing both into focus simultaneously.
This allows the photographer to validate the camera system and
evaluate details that would not be possible otherwise.
Get the kind of resolution out of your equipment that would make Ansel
Adams cry. Isolate problems, calibrate and utilize equipment at the
highest possible level of performance.
With advances in sheet film scanning (the betterscanning.com fluid
scan film holder of one) you can create high quality 4x5 images that rival
several hundred dollar fluid drum scans.
Using GG is great for composition but almost useless for finely
focused precision photography. Create images reliably and consistantly
that fully exploits the 3200dpi fluid scan resolution with breathtaking
results.
To use the plenum and loupe simply remove the Ground Glass holder.
Once Focused remove the plenum, re-insert the GG and Film Holder
(or 120/220 back) and take the picture.
Satisfaction Guaranteed or Money Back (less shipping)
Pricing Includes Full Documentation and Support
DIY fabrication of the glass and shimming it to the critical dimension.
For normal to near telephoto lenses:
$79.95 for the Loupe (default center of frame stand).
For Wide Angle Angle:
$119 for Loupe and side (of 4x5 frame) stand
$149 for Loupe, side and corner (of 4x5 frame) stand
Ready to go tested shimmed plenums
$49.95 For Glass
$49.95 For Acrylic
Tools for lp/mm Lens Measurement and Profiling
$79.95 for Microscope with Reticule and CS Target.
Transactions will be conducted via ebay and paypal.
Search for "Fine Focusing Loupe" for example listings.
Let me know you saw it here so we can customize your kit.
Shipping is $10, $20 for 4x5 plenums and $30 for 8x10 plenums
Comming soon
$349 4x5 kit with everything for the serious LF photographer
$449 8x10 kit with everything + corner stand (builds on the 4x5 kit)
8x10 plenums are glass only
Still in development
$149 Precision Film Holder (+/- .0005" aligned emulsion plane), USB Black Bag Camera, Documentation and Support
$249 Precision Film Holder (+/- .0005" aligned emulsion plane), Darkroom Night Vision Documentation and Support
$Not Priced yet - Universal upgrade to CCD fluid scanning systems
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Re: FS 4x5 Ultra Fine Focusing and Calibration
This is very interesting. I would like some more information.
Would you have any pictures or a website you can post?
I found this on ebay, is this what i would be purchasing?
Thanks
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Ian, this is a somewhat confusing post. But yes I think what he is selling is a classic focusing telescope with a calibrated reticle within the eyepiece. The magnification factor of the telescope is so high it would require viewing an aerial image only. The second part is, I think, a film holder with a clear screen but perhaps some small fiducials on the side closest to the lens. One focuses the telescope (high power loupe) on the fiducials to capture the equivalent film plane then one focuses the LF lens at the same plane. Now both fiducial and image are exactly parfocal. Next, assuming the film holder is exactly the same design as the test holder (.191 inch depth from the holder stops; or whatever) one will have captured the exact focus plane with the film holder (minus variations in the film flatness of course, which can be significant).
Perhaps the seller can confirm my interpretation as well as yours.
Nate Potter, Austin TX.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Either way, I think it's a violation of forum rules to point to an ebay listing or store in a FS or FT listing here.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
I'm afraid you could be right in some small sense, so someone needs to ask site admin for clarification and perhaps suggestions on how one might communicate facts gracefully without violating forum rules (and/or their intent).
We also need a judgment on if the "Critical Mission" nature of the site over rides mindless strict interpretation of its rules. In my providing carefully engineered solutions and techniques to show that film is still superior to digital (and its global marketing machine) I have hopefully served the greater good.
I was hoping to be up front as to how the for sale goods are transacted (on that big nasty auction site and its escrow mini-me who I really do not like - at all).
My guess is an using an URL is generally bad practice, but mention of a site would be ok as long as it has content with honest intellectual and LF forum value (as opposed to cross-posted spam messages we get in our email every day) and that this finding is recognized by members and moderators in general.
As another example, technically I've probably violated that URL concept (along with a
few other members) for yet another compelling LARGE FORMAT FORUM charter driven reason. We did point to a fluid film scanning product that has solved 90% of the problems with holding 4x5 film flat and Newton's rings on flat bed scanners. I've lost hundreds of hours to that problem, multiplied that by other film scanning LF members means it has likely been a HUGE problem in urgent need of a solution to many.
Furthermore I would like to scientifically prove here that film will be superior to digital for the rest of our lives given TWO things.
1.) We can scan LF film at its maximum resolution by holding it near perfectly flat
WITHOUT Newton's rings (hence the critical reference to the betterscanning.com
4x5 fluid scan holder - there you made me do it again)
2.) USING THE SAME TANGO DRUM SCAN FLUID MOUNTING TECHNIQUE (which can cost $300 for ONE full resolution 4x5 scan) to hold the unexposed film perfectly flat
(to +/- .0005") and have the emulsion plane within better that +/- .001" of the finely precision focused image (that pesky 4x5 plenum photography and loupe again - which is actually a microscope available with 90 degree, 75 degree and 65 degree stands for 4x5 wide angle lenses, one can get by with just 90 degree stand for a normal to near telephoto lens).
This far exceeds the repeatable capabilities of ground glass and as such is worthy of telegraphing this information (about the loupe with plenum kit and fluid scanning holder) to forum members, many who have already invested thousands into their equipment and never actually seen what it is really capable of doing and should
be able to do so at will and on a daily basis (another purpose of the forum I would
suppose).
So, is this forum a safe harbor for all things large format ? Or is this forum here so that the strictest interpretation on rules can trump all things Large Format ?
I guess we will find out. In the mean time I've got about a preview of about 60 pages of documentation, drawings and pictures that needs to go out to those who are
interested in more information. I know people embed URLs to pictures on the site
legally so I need to learn how to do that when time permits.
Cheers !
Richard
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Greetings Nate !
Quote:
Originally Posted by
Nathan Potter
Ian, this is a somewhat confusing post. But yes I think what he is selling is a classic focusing telescope with a calibrated reticle within the eyepiece. The magnification factor of the telescope is so high it would require viewing an aerial image only.
I'm replacing the 100x microscope with components that are of much more value
the the LF work flow everyone wants.
Quote:
Originally Posted by
Nathan Potter
The second part is, I think, a film holder with a clear screen but perhaps some small fiducials on the side closest to the lens. One focuses the telescope (high power loupe) on the fiducials to capture the equivalent film plane then one focuses the LF lens at the same plane.
I use a clear glass or acrylic plenum that has been shimmed to the "critical dimension"
of .190". Not only do you get mind numbing sharpness (across the entire frame) but
the film emulsion plane can be brought into sharp focus simultaneously. AN involuntary
WOW!!! comes out every time I see it.
Quote:
Originally Posted by
Nathan Potter
Now both fiducial and image are exactly parfocal. Next, assuming the film holder is exactly the same design as the test holder (.191 inch depth from the holder stops; or whatever) one will have captured the exact focus plane with the film holder (minus variations in the film flatness of course, which can be significant).
EXACTLY !!! And yes, out of the box film flatness is our worst enemy. I have a solution and its status is mentioned in another post in this thread.
Quote:
Originally Posted by
Nathan Potter
Perhaps the seller can confirm my interpretation as well as yours.
Not only are we on the same page, but we are both using big-o-loupes !!!
I'm not sure about the use of all the fiducials (both shims and ideally perfect
aligned planes) but after I post some pictures (will need to learn how and
see if they will let me) it should be much clearer.,.
All the best,
Richard
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Re: FS 4x5 Ultra Fine Focusing and Calibration
> This is very interesting. I would like some more information.
Would it be ok to send the preview information to ianmazursky.com ? I don't want to be spamming anyone by accident and I'm not sure yet about forum protocol.
> Would you have any pictures or a website you can post?
I do have a pile of pictures and I'll post some as soon as I figure out how and if the
site admin will let me. I'm a old LF guy but using this site is new to me.
> I found this on ebay, is this what i would be purchasing?
Thats an old entry level listing that I should delete since there is a much better kit offered now. For the professionals I've recently decided to recommend a kit that
(1.) Provides Mind-Blowing Focus across the entire frame (got that down).
(2.) Provides a way to expose film in perfect alignment (got that down).
(3.) Hold film so flat it as a mirror like reflection with no distortion when inspecting
the holder. Currently the problem is not the holders, even though a vacuum back
cannot achieve what I want. Its the out of the box film curl that is killing us. Even
the cheap graflex holders can hold the film at +/- .001" of where we need it. I have
a solution, it uses the same taping technique as with the drum scanner. I just need
time to prototype and fabricate a one glass septum holder and test the useage of
a black bag USB camera and/or night vision IR headset in the dark room.
4.) For those of us who do not own a drum scanner, away to do a 4x5 fluid scan on
a flat bed up to 3200dpi with film perfectly flat without newton's rings, thank you
very much betterscanning.com)
So thats it. Every LF Photographer should be able to snap and digitize a 4x5 piece
film so sharp that the end product (500MB Tiff file) would make Ansel Adams cry.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Michael Graves
Either way, I think it's a violation of forum rules to point to an ebay listing or store in a FS or FT listing here.
I'm afraid you could be right in some small sense, so someone needs to ask site admin for clarification and perhaps suggestions on how one might communicate facts gracefully without violating forum rules (and/or their intent).
We also need a judgment on if the "Critical Mission" nature of the site over rides mindless strict interpretation of its rules. In my providing carefully engineered solutions and techniques to show that film is still superior to digital (and its global marketing machine) I have hopefully served the greater good.
I was hoping to be up front as to how the for sale goods are transacted (on that big nasty auction site and its escrow mini-me who I really do not like - at all).
My guess is an using an URL is generally bad practice, but mention of a site would be ok as long as it has content with honest intellectual and LF forum value (as opposed to cross-posted spam messages we get in our email every day) and that this finding is recognized by members and moderators in general.
As another example, technically I've probably violated that URL concept (along with a
few other members) for yet another compelling LARGE FORMAT FORUM charter driven reason. We did point to a fluid film scanning product that has solved 90% of the problems with holding 4x5 film flat and Newton's rings on flat bed scanners. I've lost hundreds of hours to that problem, multiplied that by other film scanning LF members means it has likely been a HUGE problem in urgent need of a solution to many.
Furthermore I would like to scientifically prove here that film will be superior to digital for the rest of our lives given TWO things.
1.) We can scan LF film at its maximum resolution by holding it near perfectly flat
WITHOUT Newton's rings (hence the critical reference to the betterscanning.com
4x5 fluid scan holder - there you made me do it again)
2.) USING THE SAME TANGO DRUM SCAN FLUID MOUNTING TECHNIQUE (which can cost $300 for ONE full resolution 4x5 scan) to hold the unexposed film perfectly flat
(to +/- .0005") and have the emulsion plane within better that +/- .001" of the finely precision focused image (that pesky 4x5 plenum photography and loupe again - which is actually a microscope available with 90 degree, 75 degree and 65 degree stands for 4x5 wide angle lenses, one can get by with just 90 degree stand for a normal to near telephoto lens).
This far exceeds the repeatable capabilities of ground glass and as such is worthy of telegraphing this information (about the loupe with plenum kit and fluid scanning holder) to forum members, many who have already invested thousands into their equipment and never actually seen what it is really capable of doing and should
be able to do so at will and on a daily basis (another purpose of the forum I would
suppose).
So, is this forum a safe harbor for all things large format ? Or is this forum here so that the strictest interpretation on rules can trump all things Large Format ?
I guess we will find out. In the mean time I've got about a preview of about 60 pages of documentation, drawings and pictures that needs to go out to those who are
interested in more information. I know people embed URLs to pictures on the site
legally so I need to learn how to do that when time permits.
Cheers !
Richard
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Re: FS 4x5 Ultra Fine Focusing Sheet Film Preview
These are the first four pages of the sheet film document.
Out of the box sheet film curl is the biggest contributor to
softness. New Tools are to be tested soon so that near
perfect film flatness can be obtained prior to exposure.
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Re: FS 4x5 Ultra Fine Focusing Stands Preview
This is the Preview for the Loupe Stands on the 4x5
For a normal to near telephoto the 90 degree stand will do
For Wide Angle three stands are required for full frame coverage
90 degree for center of frame
75 degree for middle to side of frame
65 degree for corner of the frame
Note that full frame coverage for a wide angle 8x10 will use the
above stands plus one more for the corner.
Each stand has an arrow center line to be pointed at the center
of the lens aperture.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
So does the set up take into fact that sheet film is able to float around in the sheet film holder by as much as 0.010 on inch this depends on the age and make of the holder. It can also bow out more in the center depending on how the camera is being used. Also different makes of film are made on different thickness of stock varying about 0.003 of inch.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Whew rvhalejr! Your verbosity is really scuttling my curiosity.
Nate Potter, Austin TX.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
I have no idea what this thread is getting at but something tells me I want it. Is this a product usable on 8x10?
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
RichardRitter
So does the set up take into fact that sheet film is able to float around in the sheet film holder by as much as 0.010 on inch this depends on the age and make of the holder. It can also bow out more in the center depending on how the camera is being used. Also different makes of film are made on different thickness of stock varying about 0.003 of inch.
Greetings Richard,
Let me address each of the issues you have raised, but first a make a quick comment.
The kit is designed to let 90 percent of the geared 4x5 systems focus with awesome accuracy. It does, however, require skills to be acquired and a learning curve (training would be a great aid if available). You will have much better control and understanding of each lenses' characteristics than would other wise be possible.
> So does the set up take into fact that sheet film is able to float around in the
> sheet film holder by as much as 0.010 on inch this depends on the age and
> make of the holder.
As you have indicated, getting this finely focused precision image onto film is the biggest problem right now. The higher end kits are designed to help weed out the
poor holders from the better ones. As far as .010 of an inch goes Quick Loads and Ready loads (and out of flat holders in general) are so sloppy that better focusing will have almost no effect and are not recommend unless a lot of softness is acceptable.
> It can also bow out more in the center depending on how the camera is being used.
Yes, that is a known problem with some types of photography, 8x10 and larger formats.
Yet it is possible to tape down film (using a refined version of the betterscanning.com method) to better that +/- .001" (it will have a perfect mirror reflection with no distortion). To my knowledge, there is not a holder available yet to hold this type of septum (or precision plenum). :( sigh
Right now out of the 4x5 box film curl is killing us. If you are lucky enough to get 4x5 fuji, kodak, ilford, etc. out of the box flat (ideally within +/- .0005") and place it in a good quality film holder (flat within +/- .001") one can capture an outstanding image.
> Also different makes of film are made on different thickness of stock varying about 0.003 of inch.
Absolutely. Off the top of my head I think 4x5 films vary from about .006" to .010"
these days. 120/220 backs seem to work well (that reverse curl is known to cause problems if the film sits to long though) and is generally thinner than the sheet film.
The critical dimension will vary (as the documentation discusses) from film type to
film type. That means a dedicated precision plenum will be required for each type
of film and a validating set of bracket shots (with an incrementally differing critical
dimension for each) will be required for every system otherwise we are back to
guessing.
All the best,
Richard V. Hale Jr. (hence the unique rvhalejr user name)
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Nathan Potter
Whew rvhalejr! Your verbosity is really scuttling my curiosity.
Nate Potter, Austin TX.
Its an important technique used to avoid mis-understanding, which we have all
had to much of already.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Sizam
I have no idea what this thread is getting at but something tells me I want it. Is this a product usable on 8x10?
Yes, I have an 8x10 plenum in stock and 3 out of 4 the four loupe stands ready to go.
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Re: Image and Document Preview Available
There are about 8 .pdf pages attached to messages above that
contain information and images that describe the product in
more detail.
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Re: FS 4x5 Ultra Fine Focusing and Calibration
I have moved this to the "announcements" area as this is a product that may interest many. The "for sale" area is NOT for business to sell their wares and clearly states that. This thread should be kept to providing information about the product and related questions. Sales should be handled through the business' web site. Links to Ebay sales et all are not acceptable.
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Re: Images and Documentation Preview Loupe Stands
Quote:
Originally Posted by
Kirk Gittings
I have moved this to the "announcements" area as this is a product that may interest many. The "for sale" area is NOT for business to sell their wares and clearly states that. This thread should be kept to providing information about the product and related questions. Sales should be handled through the business' web site. Links to Ebay sales et all are not acceptable.
Attached are 4 preview pages (1,2,3 and 17 with images) of the loupe stands.
Policy Noted, Thanks,
Richard
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Re: Sheet Film Preview 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Kirk Gittings
I have moved this to the "announcements" area as this is a product that may interest many. The "for sale" area is NOT for business to sell their wares and clearly states that. This thread should be kept to providing information about the product and related questions. Sales should be handled through the business' web site. Links to Ebay sales et all are not acceptable.
Attached are four Sheet film preview pages (1,2,3,4 images and documentation)
Policy Noted. Some of the challenges posed by out of the box film curl are
mentioned here.
If you are lucky enough to be using perfectly flat film in a good quality holder
then the following does not apply. However, if the film you need to obtain
a specific look (a unique brand of C41, E6 or B&W) is curled out of the box
then the following may be of interest.
The fourth page is the most significant as it contains the reflection from
the emulsion of a 4x5 film that has been taped down to +/- .0005" using
rollers similar to drum scan mounting or that documented with the flat
bed betterscanning 4x5 film holder.
This reflection was carefully inspected for any distortion and was verified to
be mirror like. This demonstrates that out of the box film curl can be tamed.
The bad news is that the septum size in the 4x5 film holders is to small to roll
and tape mount film (it was a very time consuming experience and was very
disappointing when it failed on the very last step).
An Infrared camera and or Headset have not been tested yet for use in the
black bag or dark room.
Crafting a "film holder" to expose plenum rolled and taped film is proving to be
(as expected) extremely difficult, expensive and time consuming. Hopefully a
working prototype will eventually result in something I can use reliably without
that much difficulty.
All the Best for Now,
Richard
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Re: 4x5 Ultra Fine Focusing and Calibration
I'm still intrigued but I'm not getting exactly what your selling and how to use it. The PDFs read like a biochemistry article on the subject of focusing and film flatness, I need to see an instruction manual and an actual 1-2-3-4 step process of what it is I'd be using and how I'd use it.
Is there a glass sheet that I tape my 8x10 film onto that I slide into some sort of special film holder you provide?
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Re: 4x5 Ultra Fine Focusing and Calibration
The most logical solution to in camera film flatness would probably be a vacuum holder. But certainly not so easy to implement technically. There are small portable low vacuum devices used for picking up small parts and something like that could be adapted for the purposes of holding film flat. One would need to design a special film holder of course but I'm sure that could be accomplished. Probably not useful for Quickloads though.
But given the myriad of variables that degrade image quality under practical field conditions I would question the utility of being really fanatical about film flatness in the field. Sounds more like an exercise necessary for precision scientific photography.
In fact I used the vacuum hold down approach for the photo documentation of IC chips before features became so small as to not be resolvable with standard lenses. The film plate was specially machined with a few hundred tiny holes accessing the vacuum space under the faceplate. The vacuum space was connected to a small hose which was attached to a dry vane pump capable a few tens of mTorr vacuum. I sort of wish I had my design for the device now.
Nate Potter, Austin TX.
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Re: 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Nathan Potter
But given the myriad of variables that degrade image quality under practical field conditions I would question the utility of being really fanatical about film flatness in the field. Sounds more like an exercise necessary for precision scientific photography.
Nate Potter, Austin TX.
A working vacation for photographers can put them on a location where a once in a
lifetime money shot can be taken.
The holy grail of 4x5 photography is a 16bit fluid scan that yields >= 400 mega pixels of an image worthy of high acclaim. The end-product is mural fine art (see my profile for amusement and muse).
Quote:
In fact I used the vacuum hold down approach for the photo documentation of IC chips before features became so small as to not be resolvable with standard lenses. The film plate was specially machined with a few hundred tiny holes accessing the vacuum space under the faceplate. The vacuum space was connected to a small hose which was attached to a dry vane pump capable a few tens of mTorr vacuum. I sort of wish I had my design for the device now.
In another life I also used high vacuum equipment. I've done a prototype and the
problem encountered was that the film gets rumpled at each hole (with a hole density of about 100 and hole size of about 1/16" ... I have not tried smaller hole sizes (I do have a micro-bit set now) and higher hole densities (200 or 400 - ugh).
Rolling and taping (for film scanning) does hold film perfectly flat. But designing a
a precision holder to accept the plenum/septum is extremely problematic, expensive and time consuming.
In the end it may be less work using a well calibrated pan head with the mural being
a sequence of panels that look like windows. The experience viewing a sectional
verses a continuous display would be different. Perhaps one might be better than the other in different environments.
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Re: 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Sizam
I'm still intrigued but I'm not getting exactly what your selling and how to use it. The PDFs read like a biochemistry article on the subject of focusing and film flatness, I need to see an instruction manual and an actual 1-2-3-4 step process of what it is I'd be using and how I'd use it.
For sale are
(1) the tools and instructions for taking pictures worthy of a tango (or betterscanning.com flatbed) fluid scan (maybe as high as a giga pixel for 8x10)
(2) the tools and instructions to characterize your lenses (and entire system end-to-end) using the lp/mm MTF metric
There are some many derivative functions and improved capabilities that it is improbable that I could ever describe them all in a way that would satisfy everyone's questions.
Lets just summarize it by saying that the tools and documentation are very powerful
both optically and functionally.
Quote:
I need to see an instruction manual and an actual 1-2-3-4 step process of what it is I'd be using and how I'd use it.
The 1-2-3-4 alignment instruction document is in the works. Its purpose is for bringing the emulsion plane into perfect alignment with the lens image plane. This is done with feeler gauges, glass gauge blocks, stainless steel rule and honed digital calipers with the .0005" touch feature.
The first task is to characterize the film holders for the critical
dimension (back of camera mating surface to emulsion plane). Once critical dimension
is known a set of bracket shots is taken with plenums shimmed in .001" increments
in and on either side of the emulsion/image plane.
The focus of each shot is done with a piece of developed film you plan to use (taped
to the plenum glass in the flat emulsion/image plane at the critical dimension distance from the back of the camer), a 35x loupe and the 90, 75, 65, and 55 degree stands to cover the entire (160mm 8x10 wide angle) frame. You do not need as many stand changes for normal and near telephoto lenses.
How you bring the center, middle, side and corner of the image into fine focus is
dependent on your lens lp/mm profile characteristics, scene and your style.
The scene image will be in focus at the same time and distance as the grain in the developed films emulsion layer. These planes are aligned to one another (residing in the
same space as close as possible) and calibrated for use with a specific film thickness
and set of holders.
Once you determine which plenum shim distance works best you will shoot with that
from that point forward UNLESS the thickness of your film changes or you use a different type of film (that has a different thickness).
This instruction is a lot different from that of characterizing lens MTF lp/mm measurements (as was described in the older auction site ads).
Quote:
Is there a glass sheet that I tape my 8x10 film onto that I slide into some sort of special film holder you provide?
If your film does not curl out of the box then a standard film holder should work fine.
8x10 film does seem to have a problem with sagging because of its size and weight.
Rolling and Taping the film to a glass septum will hold the film down with mirror like
precision.
I'd love to provide such a beast (and would be very happy if anyone else did for that
matter).
The 8x10 is going to wait until the 4x5 version is complete.
The biggest issue with rolling and taping is that the glass-septum needs to be about the same size as the film holder to give the tape something to hold onto. This makes
it very difficult to design a film holder that provides plenty of tape room and enclose
the septum and dark side(s).
Precision pieces of sheet metal held together by machine screws (in a laminar fashion) seems to make the most sense at this point.
All this pain is a result of the film OEMs not making film that does not curl. If the fine
focusing methodology becomes generally accepted I would love nothing more than to
see flat film come fresh out of the pouch (box).
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Re: 4x5 Ultra Fine Focusing and Calibration
I am with Nathan on this one....
But first, i would like to applaud the OP for his tenacity in his pursuit, we need more people like this in LF :-)
Like others, I am a bit confused....but I think I finally understand the targeted goals of the products presented:
1) A method to check / correct the film plane vs. the focus plane (in our case, the gg)
2) A method to make film flat while used in a film holder, to prevent focus errors where film curl, or film buckling exist.
3) A method to place keep film flat while being scanned, and the film resides exactly on the focus plane.
Am I missing anything?
Based on this, here is a few thoughts on each of the above...
1) With most modern cameras, probably mid 80's +, makers go through extremes to keep the film plane and focus plane aligned. This is true of the better film holders as well. Often this task is confirmed and / or tweaked with lasers, which offer precision well beyond what the eye can accomplish via optics. More vintage cameras / film holders would probably benefit from the OP's system.
Keep in mind here..... if the film plane and focal plane are out of alignment, we are often saved by high f stop numbers LF uses. If LF lenses were shot at f4.0, I too would be chasing flatness down to .001".... but in non-macro photography, this issue is not so critical, such as landscapes.
Example, at f22, using .017mm for cc value (60 lp/mm), the film can vary from the focal plane by as much as +/- .015" and still acheive the desired on film resolution. This is 15x greater than .001". At f45 this "depth of focus" slop variable doubles, at f11 it's cut in half.
Also, assuming you are not photographing a flat subject (which would be a good application for the OP product), mis alignment between the film plane and focal plane greater than the amounts above, doesn't reduce resolution on the film, it simply changes the focus distance on the other side of the lens (the scene). For example, lets assume a 150mm fl lens, and you set focus at 40ft. If the film is sitting .015" further back, it will produce sharp focus at a 34ft distance. In most cases, this will rarely have any negative effects on the image recorded.... which is the beauty of LF, it really does have inherent built-in allowances for errors. Our errors of focusing are greater than this, i.e. if you tried several attempts to focus at 40ft on a ground glass, the true fl distance would vary between 35 - 45 ft... the reason is, the gg is not of sufficient resolution to enable better focus. Then toss in movement of the lens panel when locking down focus, inserting film holders, etc. Errors are everywhere.... film / gg alignment is certainly not the "weak" link in the chain. There is many weak links.
In addition, these errors are more likely to exist with lens and film standards not being square to each other, specially in older wooden vintage cameras....as wood moves over time. I would focus on these issues first.
Also a high precisioned straight edge, by a company such as starrett, combined with a high precisioned dial indicator with a roller wheel tip, at .0001" resolution, is a much simpler method IMO to pick up any errors between film plane and gg alignment. You can buy flat sheet shims in thicknesses from .001" to .1" in any increment you want, to simulate the film thickness. You can cut these shims to the film thickness you use, to make a perfect calibration between film holder and film of your choice. The shims at MSC are dirt cheap, less than $15 for a pack of multiple thicknesses. To get really picky, you need to have the focal plane lay in the middle of the grain thickness, which excludes the film base. Of course, films are close enough in thickness where this really is not a problem, unless you have a very unique application....maybe gigabyte film shot at f5.6?
2) The same numbers above, apply to film curl, or lack of film flatness. I see this being problem in two areas.... the first is, pointing the camera downward, specially with 8x10 film, the center will buckle. And secondly, roll film sitting too long in a reverse curl film holder, which there is no good solution for, other than wasting that next exposure. As for holding film flat in film holders, as mentioned previously, micro holes with a small hand pump will make the film perfectly flat, even roll film. I have this with a 6x7 RFH, and it works superb... I have not felt compelled to do this with sheet film. There has been a few makers of vacuum back film holders.... Schneider made them in 4x5 and I think Hoffman made them in 4x5 - 810.
3) Scanner film flatness. This is can be a useful tool if the scanners focal point is not on the film plane. On a flat bed scanner, I lay AN glass over my film, with Kami fluid, and all film is perfectly flat. With the consumer based scanners, this might be an issue, as I am sure QC is limited. Not sure if the betterscanning.com solution can get the film below the glass plane.
Most scanners use lenses in the f5.6 region...this again leaves sufficient Depth of Focus at the film plane, where changes of focus distances of .001" will not deliver any sharper results. The quality of the lens, the quality of the recording device, the quality of the light and electronics is what delivers a great scan. However, I am not discounting the value of confirming the lens focal plane is well aligned with film plane. I think Doug did a great job of exposing this shortcoming...
-
Re: 4x5 Ultra Fine Focusing and Calibration
Following up on the scanner calibration I mentioned above...
On Better Scanning web site, they show the improvement in a .07" focal plane alignment correction, 70x greater than the mentioned .001" correction, and even then, it's not extreme, but still worth it IMO...
http://www.betterscanning.com/scanni...advantage.html
-
Re: 4x5 Ultra Fine Focusing and Calibration
Thanks for the detailed post outlining concerns. Its a long one so it will take some
time to cover all the points raised. If you don't mind me asking I noticed the bglick
post of 16-Aug-2005 "cc vs. lp/mm as it relates to hyperfocal distance"
If that is not a good place for me to gather background information on your perspective feel free to let me know.
Quote:
Originally Posted by
bglick
I am with Nathan on this one....
...but I think I finally understand the targeted goals of the products presented:
1) A method to check / correct the film plane vs. the focus plane (in our case, the gg)
2) A method to make film flat while used in a film holder, to prevent focus errors where film curl, or film buckling exist.
3) A method to place keep film flat while being scanned, and the film resides exactly on the focus plane.
Am I missing anything?
No.
Quote:
Example, at f22, using .017mm for cc value (60 lp/mm), the film can vary from the focal plane by as much as +/- .015" and still achieve the desired on film resolution.
No.
Let me assert that "the desired on film resolution" are the same results
that Ansel Adams got at Yosemite, with the end results being a nearly ideal
400 megapixel+ image (resulting from perfectly exposed 4x5 fluid scanned film).
Also I tend to verify, test and document everything, over, and over and over .,.
This is not the first time I've heard that the focal plane envelope of optimal
focus can vary "as much as +/- .015" especially by those who use quick loads
and ready loads. I do not recommended using them because they are known to be very soft.
I thought it was obvious to most that by looking at an unexposed piece of film
in one of those "its so easy" quick and ready things in broad daylight that the
emulsion layer's reflection and distortion at the edges reminds one of a circus
mirror and the scanned results cannot be anything but laughable.
Maybe thats one complication, you are not differentiating between focal
plane flatness, incrementally moving the plane up to a distance of +/- .015
(along the normal vector) AND nasty film curl which is helical in nature,
remembers the curve of the roll it came off of and is a good example of distortion.
I can make since of the former and see that there may exist a lens lp/mm profile
(center, middle, edge and corner of frame circumscribed by the behind lens
conic section) that at a small aperture (approaching the Rayleigh limit) or
pinhole resolve an image within an envelope approaching +/- .007 .,.
Time to put on the Test Engineering hat and figure out what is going on with
this assertion these learned scientists have made.
This perhaps (though unlikely) with the condition that the center, middle,
edge and corner of frame lp/mm values are IMMUTABLE throughout that
envelope !!! (You can-not have good focus if its constantly changing as
a function of the lenses full profile).
OOOOOOOHHHHHHH !!! AAAAAHHHHHH !!! If the pinhole (tiny itsy bitsy aperture)
assertion about the envelope is true, maybe there is some use for
the Nyquist 3d 2*root(3) here, but alias, I regress (sorry,
getting back on target). For the record my working range is usually f8
through f22, ok add an fstop to eather side for less than ideal conditions.
I apologize to the f64 and up folk, I've spent way to much time in another
life modeling knife edge photon scatter in hundreds (maybe thousands) of
photo micro graphs (and 3d beam topologies, 10, 50 and 90 percent
energy lines, etc.).
Ideally this experiment will have four scopes locked on USAF targets and
instrumenting lp/mm as we rack through the normal base envelope AND
that these supposed IMMUTABLE values are recorded on film (as observed)
and easily studied in the fluid scanned generated image (our real world
work flow).
Other constraints (not mentioned, for example, could be) excellent signal
to noise ratio and perfect point resolution, modeled as slope at a point,
correlation with knife edge photon scatter, standard deviation and
(but not limited to) other generally recognised scientific and engineering
methodologies.
Well, even if I fall flat on my face its always best to tell everyone exactly
what I'm going to do, listen to any an all suggestions and complaints,
and then execute it. Let the facts be what they may.
The resolving power of a wide angle schneider lens focused on a power line
(and insulators that hold it) at 100 yards @ f22 is impressive, but seem to be
resolved at one and only one point on the rack an pinion focusing mechanism
(and NEVER behind ground glass - yes I'm a Beattie Ultra-bright addict but only
for composition of the frame as a whole).
I've got a chart somewhere about how ground glass does bad things to sharpness
so if all of your testing has been done behind GG with ReadyLoads I'd need three or
four stiff drinks before telling you the were in focus (my wife calls be a film snob
so I apologize to those who are happy with Quickloads and GG - happiness is
a good thing).
I've tested, verified and documented this over and over again and will
continue to do so. I'd love for your assertion to be true because if I could
re-create it 4x5 photography would be much simpler but alias, I am never
that lucky (sigh).,.
I'm happy for you that there are many a math model that proves this, but
I want to see the results come out of a Heidelberg Tango or Higher end
CCD scanner. Then (what ever the results) look at and maybe come up
with a little math model or two.
Now on the off chance that your assertion is correct I hope to be running
yet another test soon !!! But this time it will be with a piece of mirror
like mounted film inside a newly fabricated film holder.
Now I've really done it, let the cat out of the bag, I hope i do not get
eaten.
I'll use three different shnieider lenses (all wide angle - hopefully one will
have this magical focal plane property under normal use). I do not plan to
go above f32 but should probably use it for completeness. I'm apprehensive
about f64 because it is so dark and there are diffraction issues.
Then I'll run the film through some calibrated wooden graphic holders,
some calibrated plastic elites and then those quick ready easy load things
(choke - anything for science).
I hope I can find a lens that will give me +/- .015 inches of sloop to play with,
but I characterize those things to death although usually not at
infinity. If I cannot reproduce your assertion I'll even go as far as testing
lp/mm with big USAF targets at infinity with a wide angle, normal
and near telephoto lenses (on a foot ball field - anything for science).
Its always fun to learn something new. If there is any way to re-create the
effect described above I will quantify, qualify, study and test it to death.
But I need to actually fabricate that super flat film holder just to be on the
safe side.
All the best,
Richard
-
Re: Engineering Validation Request (Fine Focusing)
Please supply the following information
I. A lens characterization profile (lp/mm matrix) with
A.) Center, Middle, Side and Corner of lens (x axis)
B.) f16, f22, f32, f64 (the limit of pinhole size is your choice along y axis)
II. Repeat with film (or sensor) plane offsets (along the normal
vector) of .007" , .015" , .021" (more if desired).
III. State distance to target, mm size of wide angle lens and
format (6x9, 4x5, 8x10, etc).
- Use of common window glass (shimmed for offsets) is fine.
- A 35x better pocket microscope is required for somewhat accurate measurements.
- For 4x5 90, 75 and 65 degree angled microscope stands are needed.
- For 8x10 a 55 degree angled microscope stand will be needed as well.
Best Regards,
Richard
-
Re: 4x5 Ultra Fine Focusing and Calibration
Hi Rich
Thank you for the thorough response....
The only reason I mentioned cc, was how it related to focus plane alignment, NOT hyperfocal distance, which is a completely different issue, un related to the discussion here.
> 400 megapixel+ image (resulting from perfectly exposed 4x5 fluid scanned film).
Rich, through the years, the web is flooded with mis information about how much data film can hold vs. digital recording media. Here is the short version of what is possible with 4x5 film. First, we leave out the scanner, lets ASSUME it will grab every bit of resolution from the film, (regardless of the oversampling required) so that variable is removed. Now it comes down to lens aerial resolution at the f stop used and the MTF of the film. First, use color 4x5, f32. Assume the lens is diffraction limited at f32 (it never is, but lets give film its best chance)... 1500/32 = 47 lp/mm aerial resolution. Lets use the MTF for the color film at 65 lp/m.... a good avg. Combine in the time tested 1/R equation,
1/R = 1/(1/r1 + 1/r2)
R = System Resolution
r1 = Lens aerial resolution in lp/mm
r2 = Film MTF in lp/mm
The On Film Resolution (OFR) is 27 lp/mm. If you extract this out over a 90x120 mm film area (4x5), it will equal 32MP of resolution....this is nothing new, it has been tested by every major lens and film maker for years. There has been many tests that bring 4x5 color film very close to 39MP digital backs. Based on the f stop used for each, the scales can tilt in either direction.... as an example, if you shoot at f11 instead of f32, the OFR jumps to 84MP, a huge increase and will outperform the 39MP digital back, assuming the subject matter can withstand such small DOF range.
If you substitute B&W film, say TMax at 120 lp/mm vs. color, at f32 this value jumps to 49MP, and at f11 176 MP. Still no where near the 400MP you quote, and these are BEST CASE scenarios with a perfect camera system and diffraction limited optics, perfect alignment, etc. And as you know, at f11, the DOF is so small, you will be forced to shooting subjects that are flat, or at infinity. It seems all the digital vs. film comparisons I see, fail to make these associations.... the variables are so great, the resultant outcomes are "all over the map". That is the nature of optics, all variables must be considered in a comparison, and often the photo community wants short n sweet answers to everything. But anyway, hopefully this will bring some realistic MP values into play.
> This is not the first time I've heard that the focal plane envelope of optimal
focus can vary "as much as +/- .015"
This is based on ONE single f stop.... the depth of focus principle of the film plane has been an accepted optical principle for over 100+ years. As you know, Depth of focus is a function of f stop only, not fl. As f stop increases, so does Depth of Focus, hence the amazing viability of LF photography...
As for film curl in sheet film... I personally have never seen it in the thousands of sheets I have used... and when I insert film into my film holders, the edges are held very secure, forcing flatness, spcially in 4x5 which is small enough to prevent bulges, unless you point the camera downward. I have shot quickloads for years, and never had a problem with what you describe, maybe I got lucky batches.... Fuji understands this problem very well... But to be clear, I never shoot LF at f8 either, where any of the problems you mention could surface. So the application is significant factor here ....
> Time to put on the Test Engineering hat and figure out what is going on with
this assertion these learned scientists have made.
Grab any books on photo optics and read about Depth of Focus.... its a simple optics equation that makes perfect sense. Think of it as Depth of Field at the film plane. But more importantly, if you are shooting a scene with depth, the mis alignment issues do bring different parts of the scene into more/less focus...and with a lot of depth, its impossible to evaluate this after the exposure....unless the error was soooo great it might be obvious. The only example I can think of, is extreme curl in roll film, from sitting on a reverse curl, 180 deg roller too long.
> This perhaps (though unlikely) with the condition that the center, middle,
edge and corner of frame lp/mm values are IMMUTABLE throughout that
envelope !!! (You can-not have good focus if its constantly changing as
a function of the lenses full profile).
the examples above, assume consistent aerial resolution on the entire film circle radius. At high f numbers, such as LF...this is decent assumption...as stopping down tends to level the playing field.... but the center of any spherical optic will always be sharpest. So you can reduce the numbers I provided above by 20% to be safe....again, in most cases, we are trying to get in the ballpark.... If you look at Schneiders Website, you can get the MTF curves of all the lenses made, and see radius performance levels out at high f stops. But certainly not true at f5.6, which LF lenses are never optimized for, except the new digital lenses, which can be used for RF or digital backs.
> For the record my working range is usually f8 through f22,
LF lenses will perform very poorly at f8.... these lower f stops are used to produce enough light on the ground glass to compose the image - otherwise, they would not be offered on the lens, they are not for exposing. Most all 4x5 and 810 lenses are optimized in a tight 2 f stop range..... shorter fl's in the f16-22 range, and the mid fl lenses, in the f22-32 range, and the longer fl's in the f45 range. The abberations at f8 on 98% of LF lenses are so degrading, you often can produce a better image at f64. There is a few rare exceptions, and these are very new SSXL designs and a few others, whereas in the short fl's they can be used at f11. But regardless, at f8 - f11, you are reduced to shooting flat subjects or infinity to hold resolution..
You seem to be in agreement with focus error being a necessary evil of LF in general. And of course, the big issue not mentioned is, when you focus at f5.6 and then stop down to f45 to shoot, you have shifted the focus plane, another weak link in the chain here, specially when considering the alignment precision you are pursuing. In other words, no matter how perfect the alignment is in all aspects of the camera, the shortcoming will be the inability to focus precisely, and the inability to focus well enough at higher f stops, due to lack of light. Of course, this is not justification for the rest of the camera system to be completely out of alignment.... but it demonstrates why ultra precision film/gg alignment will often not improve the final OFR.
> If I cannot reproduce your assertion I'll even go as far as testing
lp/mm with big USAF targets at infinity with a wide angle, normal
and near telephoto lenses (on a foot ball field - anything for science).
I have tested all types of lenses for many years.... and its truly remarkable when the set-up is right, how it confirms the math and often the MTF values the makers provide. This is for both film and digital. keep in mind, a football field is extreme. You don't need targets this far from the lens. Chris Perez has done an excellent job testing lenses of all types, I am sure you have seen his web site? His results concur with everything I have ever tested.
http://www.hevanet.com/cperez/testing.html
Thank you again Chris for being such a big supporter of the LF community.
To be clear Richard, I am not knocking your desire to achieve the flatness you desire. And in some extreme shooting situations, I am sure it will be very helpful, such as f5.6 - f8 doing reproduction work of 2d subjects. But using common LF lenses, which probably 90% of them in existence, are designed to be shot at f22 - 45, their is such a large inherent slop factor, in the end...the means don't justify the ends. If you follow this forum, you will see this issue surface every few years.
I think a more obvious area of attack to improve image quality is the squareness between the front and back standards, specially with short fl lenses shot at f8 - f16. As slight un-squareness presents huge variances in the focal plane vs. the film plane. Of course, this problem is magnified in 810 vs. 45.
Comments on your 2nd post....
A good test, to reduce testing time, is to first test the lens with a digital back. I have a set-up that enables a DSLR to mount as a film holder. This gives you a "sure" flat recording plane, and a great focus mechanism, as I use after-market focusing screens sold by Katz Eye Optics - with these screens you can really nail the focus to the levels of accuracy you desire. Now you have a basis to compare film with... Digital recording is subjected to the same 1/R curse....(which is Nyquist theory in action) of course, SWA and even some WA lenses will not perform well with most digital sensors, as this still a weak link in digital sensors....
Anyway, we all are interested in your findings.....
-
Re: 4x5 Ultra Fine Focusing and Calibration
Not that long ago, I exchanged info on line with a guy who made a DIY 4x5 vacuum holder. It didn't look that hard to do. It turns out that a rubber bulb pipette bulb has enough suction. I no longer have the contact info, but a little Googling should turn it up.
-
Reply: 4x5 Fine Focusing and Calibration Part I of II
We are running out of space so I've had to leave some of the good stuff
behind (sigh) :(
Quote:
Originally Posted by
bglick
Hi Rich
> 400 megapixel+ image (resulting from perfectly exposed 4x5 fluid scanned film).
Lets use the MTF for the color film at 65 lp/m.... a good avg. Combine in the time tested 1/R equation,
1/R = 1/(1/r1 + 1/r2)
R = System Resolution
r1 = Lens aerial resolution in lp/mm
r2 = Film MTF in lp/mm
The On Film Resolution (OFR) is 27 lp/mm. If you extract this out over a 90x120 mm film area (4x5), it will equal 32MP of resolution....this is nothing new, it has been tested by every major lens and film maker for years. There has been many tests that bring 4x5 color film very close to 39MP digital backs. Based on the f stop used for each, the scales can tilt in either direction.... as an example, if you shoot at f11 instead of f32, the OFR jumps to 84MP, a huge increase and will outperform the 39MP digital back, assuming the subject matter can withstand such small DOF range.
If you substitute B&W film, say TMax at 120 lp/mm vs. color, at f32 this value jumps to 49MP, and at f11 176 MP. Still no where near the 400MP you quote, and these are BEST CASE scenarios with a perfect camera system and diffraction limited optics, perfect alignment, etc. And as you know, at f11, the DOF is so small, you will be forced to shooting subjects that are flat, or at infinity. It seems all the digital vs. film comparisons I see, fail to make these associations.... the variables are so great, the resultant outcomes are "all over the map". That is the nature of optics, all variables must be considered in a comparison, and often the photo community wants short n sweet answers to everything. But anyway, hopefully this will bring some realistic MP values into play.
3200ppi is the recognized maximum for high end photography (I've seen it from those way more accomplished than I).
3200ppi^2 == 10 Megapixels per inch of film
4x5 == 20 inches (its not really 4x5)
20 X 10 Megapixels == 200 Megapixels (full color RGB ones, 16bit color depth)
The 39MP digital backs are actually Monochromatic photo sites, 1 pixel is
ether Red, Blue or one of two Greens.
So we can call the 39MP what is is a full color 10MP camera with a bit level depth of 12 (in true kodak fashion you really have to dig to find that one).
Using Monochromatic Digital Back accounting the film 200 RGB Megapixel is actually
600 Mega Monochromatic (less 25% for lack of two greens) is 450 Mega pixels based on monochromatic rgbg photo sites. But 400 seems close enough to make the point.
However, Bob, for general public consumption I defer to your wisdom and stand corrected.
Quote:
Originally Posted by
bglick
> This is not the first time I've heard that the focal plane envelope of optimal
focus can vary "as much as +/- .015"
This is based on ONE single f stop.... the depth of focus principle of the film plane has been an accepted optical principle for over 100+ years. As you know, Depth of focus is a function of f stop only, not fl. As f stop increases, so does Depth of Focus, hence the amazing viability of LF photography...
Admittedly Scheimpflug is out of scope for me. f8-f16 is good for the 35mm or DSLR spotmeter, f11-f22 for 120/220 backs on 4x5, and f32 is good on a sunny day but freezing movment becomes problematic for me.
I only find use for a limited number of movements and as you probably know use the
4x5 more like a press camera than what is described in bobwheeler.com/photo/ViewCam.pdf
Quote:
Originally Posted by
bglick
As for film curl in sheet film... I personally have never seen it in the thousands of sheets I have used...
I need to buy my film from you and/or other kind souls on the forum who are
well connected. Having flat film out of the box makes things much easier.
Still, I'd like you to shoot the mirror reflection of your unexposed flat film in a
holder and post it here (like i did with the roll and tape mount example). Carefully
inspect for any distortion or departure from what should be a mirror like image.
Trust but verify.
Quote:
Originally Posted by
bglick
> Time to put on the Test Engineering hat and figure out what is going on with
this assertion these learned scientists have made.
Grab any books on photo optics and read about Depth of Focus.... its a simple optics equation that makes perfect sense. Think of it as Depth of Field at the film plane.
Depth of Focus has some value (probably much more to view camera purists), but I have to to resist relying on it as an excuse to not shim the plenum so as to
bring Image Focus into the Emulsion Plane.
I glady leave the exploitation of Depth of Focus (and the lack of any need to
shim their plenums, or calibrated their film holders) to View Camera True Believers.
> For the record my working range is usually f8 through f22,
Quote:
Originally Posted by
bglick
LF lenses will perform very poorly at f8.... these lower f stops are used to produce enough light on the ground glass to compose the image - otherwise, they would not be offered on the lens, they are not for exposing. Most all 4x5 and 810 lenses are optimized in a tight 2 f stop range..... shorter fl's in the f16-22 range, and the mid fl lenses, in the f22-32 range, and the longer fl's in the f45 range. The abberations at f8 on 98% of LF lenses are so degrading, you often can produce a better image at f64. There is a few rare exceptions, and these are very new SSXL designs and a few others, whereas in the short fl's they can be used at f11. But regardless, at f8 - f11, you are reduced to shooting flat subjects or infinity to hold resolution..
I love shooting f32 or f45 if there is enough light and lp/mm is still very good for
that lens. However, if I want to pull detail out of shadows (for HDR) f8 can look
pretty good as the low end of a Bracket Sequence (f8, f16, f32)
Quote:
Originally Posted by
bglick
You seem to be
in agreement with focus error being a necessary evil of LF in general. And of course, the big issue not mentioned is, when you focus at f5.6 and then stop down to f45 to shoot, you have shifted the focus plane, another weak link in the chain here, specially when considering the alignment precision you are pursuing. In other words, no matter how perfect the alignment is in all aspects of the camera, the shortcoming will be the inability to focus precisely, and the inability to focus well enough at higher f stops, due to lack of light. Of course, this is not justification for the rest of the camera system to be completely out of alignment.... but it demonstrates why ultra precision film/gg alignment will often not improve the final OFR.
No. In the hands of someone like yourself the fine focusing kit will allow you to
reach heights you never thought possible. Use it a few times and you will not
want to part with it.
I think .001" (as a goal) is just good precision (thats why my fan club is in Germany) leaving the other .014" to be used by the view camera photographer as they see
fit.
Quote:
Originally Posted by
bglick
I have tested all types of lenses for many years.... and its truly remarkable when the set-up is right, how it confirms the math and often the MTF values the makers provide. This is for both film and digital. ... Chris Perez has done an excellent job testing lenses of all types, I am sure you have seen his web site? His results concur with everything I have ever tested.
http://www.hevanet.com/cperez/testing.html
Thank you again Chris for being such a big supporter of the LF community.
He is legendary.
Quote:
Originally Posted by
bglick
But using common LF lenses, which probably 90% of them in existence, are designed to be shot at f22 - 45, their is such a large inherent slop factor, in the end...the means don't justify the ends. If you follow this forum, you will see this issue surface every few years.
No. You View Camera Folks love to work in that sloop. After you have framed
the perfect shot, whip out the clear class and scopes. Photography will never be
the same for you. You will become even more of a devotee than you are now.
Who knows, maybe you will be the first one to send one back. I'd send you
a bottle of wine if that turns out to be the case.
Quote:
Originally Posted by
bglick
I think a more obvious area of attack to improve image quality is the squareness between the front and back standards, specially with short fl lenses shot at f8 - f16. As slight un-squareness presents huge variances in the focal plane vs. the film plane. Of course, this problem is magnified in 8x10 vs. 45.
We just need a piece of metal with fine set screws to hold the glass plenum
to calibrate it (achieve best possible focus). Then ether the film holders
or camera back could be shimmed. With some higher end cameras its probably
best to have it serviced at the factory.
Ill continue in the next post ;)
-
Re: Reply: 4x5 Fine Focusing and Calibration Part II of II
Continued from previous post
QUOTE=bglick;425233]
Comments on your 2nd post....
A good test, to reduce testing time, is to first test the lens with a digital back. I have a set-up that enables a DSLR to mount as a film holder. This gives you a "sure" flat recording plane, and a great focus mechanism, as I use after-market focusing screens sold by Katz Eye Optics - with these screens you can really nail the focus to the levels of accuracy you desire. [/QUOTE]
Great !!! You have the 90 degree line of sight dialed in. But Inspecting through
clear glass at 75 and 65 degrees (for 4x5) will take you to another level of understanding.
As you may already know my work is based on the Charles Sleicher manuscript
that accompanied his lens test chart. His work was the first I came across
that described using a microscope in the film plane to measure lp/mm and give
the 500 lp/mm value for the SLR 50mm f1.8 lens formula.
I verified that with my own microscope and published the picture a long time
ago (the site is long gone). 500 lp/mm is an amazing thing to see . Sadly, many seem to dislike the idea as well as micro-photographs, the fact an alternative
method might exist seemed to annoy a lot of people. I'm Agnostic, its just another tool, maybe helpful for some things and certainly not a solution for everything.
Quote:
Originally Posted by
bglick
Now you have a basis to compare film with... Digital recording is subjected to the same 1/R curse....(which is Nyquist theory in action) of course, SWA and even some WA lenses will not perform well with most digital sensors, as this still a weak link in digital sensors....
Anyway, we all are interested in your findings.....
Thanks for all the kind words and help with view camera, of which I use but a small
portion of their capabilities. Even though I'm in love with the speed graphic
as a piece of functional art and as iconic symbol for photography in Hollywood
to this day I really respect and appreciate the works created by the large
format view camera community.
I wish people could be nicer (they can get really nasty flame wars going on other
sites) but really appreciate constructive criticism with regard to technique and
theory without which I could never appreciate others fine work as it should be
and (hopefully) improve my own work.
I should consider posting my first manuscript in this forum as it has the disclaimers
in it as to the basic capabilities of the fine focusing kit. Simple things like
I'd rather be caught dead than using Anti-Newton Glass. There is a place for diffusion
filters but AN is something to be avoided (like really gross way over the to wedding pictures - which there are exceptions and I plead guilty to).
Shooting film is very expensive for me right now, so calibrating film holders, plenums and finding a way to hold film flat should save some money that otherwise might be wasted on developed film that is to soft (shooting with a good lens at is highest resolution).
High precision was turned to for the hard, cold straight line architectural look and emulation focusing what Ansel Adams work in Yosemite looks a lot like.
The diffuse soft look is another style I like some times characterized as a glamor,
high emotional image or artsy look (vignetting seems to work well with that).
Whats wonderful about sublte diffusion in Digital and Analog is that it can
increase the signal to noise ratio by a significant amount but
have little or no effect on the smallest detail (as verified by 35X magnification
before shooting).
If You Like very subtle diffusion (with portra and the 65mm SA) and want to
see a similar effect with a digital back magnification at the image plane is a
really big help in choosing the perfect on lens diffuser.
My feeling is that flat film (from connected sources) standard film holders,
calibrated at +/- .002" for 4x5 and +/- .004 for 8x10, clear plenum
shimmed to the same is probably fine for most view camera photographers
out there.
But for Speed Graphic Press Photography fans who appreciate the "brutal
sharpness" found in those photos or exacting resolution in landscape or
architecture may be worthy of a bit more effort as the maximum fstop
for sharpness is (if I understand this correctly) at a point where the
Image Plane envelope of good focus about the emulsion plane approaches
zero.
This is what I have observed up until now. But if the prototype film holder
for forcing film to be flat does no better than a stock (yet calibrated) film
holder with flat film from connected sources then that would be far easier
for me.
And if I do find extreme instances where this is the case I would not recommend
it to anyone unless they found themselves in the same predicament.
So much work to do and so little time (sigh).
Bless us one and All,
Rich Hale (haven't used that one in a long time).
-
Re: 4x5 Ultra Fine Focusing and Calibration
> 3200ppi is the recognized maximum for high end photography (I've seen it from those way more accomplished than I).
Yes, 3200 dpi, / 25.4 / 2 = 63 lp/mm to film, this is possible, BUT ONLY at f2.0 and wider, and that is not reality in LF photography. Run the 1/R equation, it will become quite clear.
I do have some f2.0 lenses that resolve 65 lp/mm, but of course, they are 35mm lenses, they are also "nearly" diffraction limited, and have 18 elements, and weigh 8 lbs, with 100mm wide aperture. 35mm lenses condense their resolution into a tiny projected image circle. LF lenses create large image circles, diluting the resolution over the image circle. When you combine this with the fact LF lenses are only designed at high f numbers, the aerial resolutions favor small formats, in a big way....
1500 / f2 = 750 lp/mm aerial resolution for 35mm lens
1500 / f22 = 68 lp/mm aerial resolution, typical 4x5 application
1500 / f45 = 33 lp/mm aerial resolution, typical 810 application.
As you can see, the 35mm delivers 23x more resolution than the 8x10 lens.
LF gains resolution through film size, unfortunately it can not also hold the optical gains of smaller formats as well - which is the premise you are suggesting. LF looses terribly in aerial resolution due to lenses designed to be used at high f stops. Have you ever seen a diffraction limited f2.0 LF lens that covers 8x10? So you have taken the attributes of tiny formats, such as 35mm high end optics, and are trying to combine them with LF film..... we can only wish n pray that such phenomena as this can be played out in the real world...
The rest of your math I agree with, once you plug in realistic ppi numbers for LF. The 3200 ppi example you offer is real, for 35mm camera systems, using the BEST optics ($6k+ lenses) and Velvia color film. In which case, as you suggest, 10MP per square inch is a fair representation of 35mm systems, which when extracted out to 24x36mm format, is equal to 15MP. This is well accepted. Using lower grade color film, of course the number would be closer to the 10MP value. However, you can NOT extrapolate this to LF, before using 1/R as the basis of what can be resolved in that format.
So, using a realistic 27 lp/mm to film, you will produce 27 x 25.4*2 = 1372^2 = 1.8MP per sq. inch, * 20 sq in (4x5) = 37MP. A huge discrepancy from your original 400MP claim, and your more recent claim of 200MP. Of course, you must take each case on its own merits as all LF lenses and film change the variables, but none of these variables will rock the results to achieve your claims. And most importantly, the f stop you shoot at is still the most critical factor for LF. I hope this makes it clearer.
> The 39MP digital backs are actually Monochromatic photo sites, 1 pixel is
ether Red, Blue or one of two Greens.
Agreed, but in the end, this has proven to be a very small liability, vs. if each pixel could read R,G,B. This is accounted for in many comparisons when Bayer sensors are compared to full RGB sensors such as Foveon. The predicting algorithms are so good, the net effect this, might loose 15 - 30% of the pixel count, based on the colors of the subjects. 20% is a good average.
> So we can call the 39MP what is is a full color 10MP camera with a bit level depth of 12
You are over simplifying here. If you want to learn more about this, google the Foveon sensors, which record RGB at each pixel site. They have been compared to Bayer sensors many times, and the "net gain" is MUCH less than the simple math suggest. Part of the reason is, each sensor can sense parts of other colors, as many colors are mixes of other colors. (an abbreviated position on a very involved issue) Of course, this is why a Bayer sensor resolves best with B&W targets, and it resolves much worse with red targets.... all this is well documented.
Bit depth, is a completely different issue..... it determines the levels of tonality within a color. Although there is a slight relationship between bit depth and resolution, its small vs. the major gaps we are dealing with in this discussion.
> Using Monochromatic Digital Back accounting the film 200 RGB Megapixel is actually
600 Mega Monochromatic (less 25% for lack of two greens) is 450 Mega pixels based on monochromatic rgbg photo sites. But 400 seems close enough to make the point.
I have addressed most of this above. However, if you want to implement the pixel color issue into play..... you can reduce the MP of the backs by....
30% whens shooting Red and Blue Targets
15% when shooting Green Targets
0% when shooting B&W targets.
If you have a digital camera with some good lenses, you can test this by shooting a nice array of color targets, and you will see, after 1/R, these reductions above represent the reality of digital recording of Bayers sensors, using at a min. high end DSLR... we are not comparing a $50 P&S digital camera here. (just to be clear) So a good avg. is 20%... again, we are trying to get in the ballpark in this discussion, i.e. somehow tame 600MP claims down to 40MP... .then one can deal with the nitty gritty details. Of course, these discussions have been going on for years on the internet, most of this is old news.
> Admittedly Scheimpflug is out of scope for me. f8-f16 is good for the 35mm or DSLR spotmeter, f11-f22 for 120/220 backs on 4x5, and f32 is good on a sunny day but freezing movment becomes problematic for me.
Scheimpflug has not entered into this discussion, not sure why you mention it.... and as for f stops, I am not following your logic here. A 35mm camera that requries 5.6 for sufficient DOF, will use f11 on MF, f22 on 4x5 and f45 on 810.... maybe what you wrote above was a typo? Simple rule, you double the format size, you double the f stop for the same DOF.
> Depth of Focus has some value (probably much more to view camera purists), but I have to to resist relying on it as an excuse to not shim the plenum so as to bring Image Focus into the Emulsion Plane.
Yes, both camera manufacturers, film holder manufacturers have done this for us, hence why there is international standards for all these issues. Unless something fell out of alignment, or you are dealing with early 1900's cameras with modern film holders, Depth of Focus at the film plane will resolve any small errors, as suggested in my previous post, by a factor of 15 - 20x, vs. the .001" you are shooting for.
end of part I
...
-
Part II
Part II
> I think .001" (as a goal) is just good precision (thats why my fan club is in Germany) leaving the other .014" to be used by the view camera photographer as they see fit.
As you can tell, I am a stickler for precision. But the precision has to have a desired end effect. I think you are overlooking the fact these issues have been addressed for many years and have surfaced in the products we use....then, Depth of Focus is the reason more precision will not benefit the final product, UNLESS, you have defective equipment, or you are shooting at f8 or wider on LF.... not sure if I explained this well enough...
> No. You View Camera Folks love to work in that sloop. After you have framed
the perfect shot, whip out the clear class and scopes. Photography will never be
the same for you.
you are missing the point Richard. The "slop" is inherent in the physics behind the optics. The slop allowed in the system, is a result of the principles behind Depth of Focus at the film plane. This is optics fundamentals at play, you have confused this with slopy users, slopy cameras, etc. You are trying to take a tolerance set by Depth of Focus, for example of +/- .015", whereas inside this tolerance range, there is no gains to be had, and you are trying to tighten that tolerance to +/- .001". You are ignoring the fact optical principles of Depth of Focus will not allow added gains with tighter tolerances. However, your system would be useful for fast 35mm lenses.
> As you may already know my work is based on the Charles Sleicher manuscript
that accompanied his lens test chart. His work was the first I came across
that described using a microscope in the film plane to measure lp/mm and give
the 500 lp/mm value for the SLR 50mm f1.8 lens formula
BTW, Charles makes a great lens test chart if anyone is interested, and its only about $35. Lets address this 500 lp/mm issue. First, a diffraction limited f1.8 lens can deliver 1500/ 1.8 = 833 lp/mm max. I am using 1500 for an avg. of the color spectrum. But in reality, NO lens at f1.8 at 50mm fl is diffraction limited, its at best, 50% of this value.... so these numbers are just tooo high. The only "near" diffraction limited lenses need to be in the very long fl's, 200mm+, as at these fl's they do not have to contend with the sharp ray angles the normal to WA lenses deliver. This is another optical fundamental shortcoming that will not be overcome in our lifetime. This is why high end refracting telescopes are usually diffraction limited, as their fl's are 500mm+, and yet they only contain 2 or 3 elements, and yet a Zeiss WA lens will have 17 elements and will never be a match to the resolution of a telescope. Sad, but true, photography puts very hardcore constraints on optics designs.
> 500 lp/mm is an amazing thing to see .
The human eye can see a max. of about 1 arc minute. To translate this into lp/mm, requires introducing distance....which can vary. But regardless, the best eyes can resolve 7 - 10 lp/mm, avg. eyes 3 - 5 lp/mm and aging eyes 1 - 3 lp/mm. Even trying to view 500 lp/mm under magnification, it would require for an avg eye, 100x magnification to produce 5 lp/mm at the eye...or about a 2mm fl eyepiece.... is this what you did?
> Shooting film is very expensive for me right now, so calibrating film holders, plenums and finding a way to hold film flat should save some money that otherwise might be wasted on developed film that is to soft (shooting with a good lens at is highest resolution).
IMO, using optics to check alignment is overkill and has too many variables that can produce poor results. You can accomplish film flatness and alignment much easier with precision straight edges and dial gauges. Since they are precisioned so fine, and in mass use in the metals working fields, you can acquire this gear cost effectively, not rely on all the diffraction low cost optics present, and also remove the human eye from the equation, as everyone does not have perfect vision. You also have the big factor of what focus plane the optics itself is focusing on. (too long to discuss here)
I have used a high precision straight edge and .0001" dial gauges and checked all my LF cameras and film holders through the years.... all of them were remarkably accurate, no shims required anywhere. Granted most of my gear was bought new in the past 10 years, I only use Toyo film holders. My point is, I think there is better methods to accomplish your task.... i.e. more accurate, similar cost, and repeatable and eliminates the human vision variable.
> My feeling is that flat film (from connected sources) standard film holders,
calibrated at +/- .002" for 4x5 and +/- .004 for 8x10
i addresses this in my previous post.... these numbers are overkill for the common f stops used in LF photography...
> And if I do find extreme instances where this is the case I would not recommend
it to anyone unless they found themselves in the same predicament.
Rich, I am sure your intentions are sincere, which is often the case within the LF community. I hope you don't find my comments offensive in anyway. This is tricky stuff, and many people can fool themselves when it comes to these issues, they are complex. I deal a lot with optical engineers and have been involved in designing many optics, as well as being a life long photographer. Sometimes knowledge in these areas is hard to find, at any level. And with the internet being filled with more mis information vs. useful information, it sometimes makes it more difficult to get to the right answers to a given task. hopefully I have shed some light on the work you plan to do.
-
Re: 4x5 Ultra Fine Focusing and Calibration
Several years ago, I posted a graph I made that demonstrated the resolution potential between the formats. I should probably update it, as when I made it orginaly, the pixel counts that would finaly surface were different vs. what I used. But regardless, it shows the film comparisons as well.
http://www.pbase.com/bglick/image/50899836/large
You can see, at infinity focus, last set of bars to the right, the 810 format holds a substantial resolution gain over all the other formats, including digital.
However, the bars next to the infinity bars show the formats when the image required a relatively high f stop to accommodate depth of field in the image. In which case, MF, 45 and 810 are relatively close to each other in terms of net resolution when comparing equal size final prints. This demonstrates what I referred to earlier.... there can be NO generalizations made when comparing format sizes, OFR, film vs. digital, etc. All things must be taken into consideration for each comparison. The variables have a radical change on the results / comaprisons.
An extreme example.... if you view the last set of blocks, (infinity focus), you can see 810 will out resolve 35mm, 13MP, (at the same size final output) by 23/4 = 6x. But in the next set of bars, where equiv. f stops are used for each format due to DOF, the 810 can only out resolve 13MP at 10/4 = 2.5x. All these charts assume color film using targets of a contrast ratio that yield 80 lp/mm MTF.... (Velvia)
While this discussion brought in 35mm lenses (albeit their performance was confused with LF lenses) .... today, these 35mm optics are beyond remarkable. While LF lenses are, and always have been somewhat limited by diffraction, 35mm lenses were not limited by diffraction - so we continue to see tremendous advances in optical quality... they just keep improving. Consider a very low contrast subject like this link below.... and hand-held at 200mm.... with a low end DSLR, and yet it can resolve like this?? And for a $900 lens ?... truly amazing...
http://www.pbase.com/bglick/image/105272354
-
Re: 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
bglick
> 3200ppi is the recognized maximum for high end photography (I've seen it from those way more accomplished than I).
Yes, 3200 dpi, / 25.4 / 2 = 63 lp/mm to film, this is possible, BUT ONLY at f2.0 and wider, and that is not reality in LF photography. Run the 1/R equation, it will become quite clear.
Is this the same 1/R equation found in the Appendix of this manuscript ?
diglloyd.com/diglloyd/free/sleicher-res-chart/SleicherResChartInstructions.pdf
Quote:
Originally Posted by
bglick
35mm lenses condense their resolution into a tiny projected image circle. LF lenses create large image circles, diluting the resolution over the image circle. When you combine this with the fact LF lenses are only designed at high f numbers, the aerial resolutions favor small formats, in a big way....
1500 / f2 = 750 lp/mm aerial resolution for 35mm lens
1500 / f22 = 68 lp/mm aerial resolution, typical 4x5 application
1500 / f45 = 33 lp/mm aerial resolution, typical 810 application.
As you can see, the 35mm delivers 23x more resolution than the 8x10 lens.
If 33lp/mm is all one can get out of 810 then I'd guess (never have used one
although they themselves are beautiful functional art) that any kit to help
sanity check alignment and perform calibration would be futile if not wasted
effort :(
Quote:
Originally Posted by
bglick
LF gains resolution through film size, unfortunately it can not also hold the optical gains of smaller formats as well - which is the premise you are suggesting. LF looses terribly in aerial resolution due to lenses designed to be used at high f stops.
Suggestion in the context of an avid photographer wondering "why or why not".
Not in any Engineering sense UNLESS it holds up under repeatable testing.
Quote:
Originally Posted by
bglick
The rest of your math I agree with, once you plug in realistic ppi numbers for LF. The 3200 ppi example you offer is real, for 35mm camera systems, using the BEST optics ($6k+ lenses) and Velvia color film. In which case, as you suggest, 10MP per square inch is a fair representation of 35mm systems, which when extracted out to 24x36mm format, is equal to 15MP. This is well accepted. Using lower grade color film, of course the number would be closer to the 10MP value. However, you can NOT extrapolate this to LF, before using 1/R as the basis of what can be resolved in that format.
No, I'm NOT extrapolating this to anything larger than 4x5 (not 810 or ULF with which I have no experience). Arguably some do not even consider 4x5 LF and when I use a 120/220 back ***OR*** 2x3 format I'm not sure I am either.
Anyone would have a hard time convincing me that Ziess, Scheinder, Nikon APO
lenses versions that rock at 2x3 should not at least be carefully examined in the 4x5
versions.
Hopefully it is recognized that a 2x3 lens with movement will have none at 4x5
and that those who do not consider "Ariel" or lack of movement LF Photography
are not offended.
To me 4x5 is sort of a a miniature LF and the number value of 68 lp/mm seems within the realm of possibilities (maybe dead on for your high quality film, equipment and experience).
That does not mean that it should not or cannot be subjected to repeatable
testing for potentially higher values.
Furthermore, it would seem prudent for little people like my self to sanity check our 1949 vintage gear (yes I rebuilt the focal plane shutter by hand and am proud of it and the history that it represents).
Quote:
Originally Posted by
bglick
So, using a realistic 27 lp/mm to film, you will produce 27 x 25.4*2 = 1372^2 = 1.8MP per sq. inch, * 20 sq in (4x5) = 37MP. A
from your original 400MP claim, and your more recent claim of 200MP. Of course, you must take each case on its own merits as all LF lenses and film change the variables, but none of these variables will rock the results to achieve your claims. And most importantly, the f stop you shoot at is still the most critical factor for LF. I hope this makes it clearer.
Sorry if if my huge discrepancy offend anyone, I have no vested interest in digital backs, really expensive photo equipment, etc.
And no (as indicated above) anything less that 70lp/mm is not what I'm generally looking for is high resolution. Any Validation Engineering lp/mm results that do not hold up to a microscope sweeping through the emulsion plane should be called into doubt as a matter of generally accepted good scientific practice.
In another life I worked on a project called the Hubble Space Telescope. No way
would NASA allow us to open up the roof and check the optics because they
knew better. So it went up on orbit and could not focus on a damn thing.
I WILL NEVER BE PUT THAT POSITION AGAIN !!!
The problem many well paid Engineers at large camera and photography related firms
is this. The Microcope and Stand setup to quantify lens resolution has become
very cheap. Anyone can outfit a lab for as little as $3000 (probably a tenth of
what some hear drop on thier systems) and verify marketing claims (unless they
have a closed black box system). This means the numbers can now be easily
verified - should be no problem if they are accurate an repeatable.
So why all the fuss about a little verification and validation ? Beats the hell out of me .,.
Quote:
Originally Posted by
bglick
> The 39MP digital backs are actually Monochromatic photo sites, 1 pixel is
ether Red, Blue or one of two Greens.
Agreed, but in the end, this has proven to be a very small liability, vs. if each pixel could read R,G,B. This is accounted for in many comparisons when Bayer sensors are compared to full RGB sensors such as Foveon. The predicting algorithms are so good, the net effect this, might loose 15 - 30% of the pixel count, based on the colors of the subjects. 20% is a good average.
> So we can call the 39MP what is is a full color 10MP camera with a bit level depth of 12
You are over simplifying here. If you want to learn more about this, google the Foveon sensors, which record RGB at each pixel site.
They have been compared to Bayer sensors many times, and the "net gain" is MUCH less than the simple math suggest. Part of the reason is, each sensor can sense parts of other colors, as many colors are mixes of other colors. (an abbreviated position on a very involved issue) Of course, this is why a Bayer sensor resolves best with B&W targets, and it resolves much worse with red targets.... all this is well documented.
I respectfully disagree with the Foveon citation. The only valid comarison for
engineering purposes are the photon-multipliers used in Heidelberg Tango Wet
Scans.
Quote:
Originally Posted by
bglick
Bit depth, is a completely different issue..... it determines the levels of tonality within a color. Although there is a slight relationship between bit depth and resolution, its small vs. the major gaps we are dealing with in this discussion.
> Using Monochromatic Digital Back accounting the film 200 RGB Megapixel is actually
600 Mega Monochromatic (less 25% for lack of two greens) is 450 Mega pixels based on monochromatic rgbg photo sites. But 400 seems close enough to make the point.
I have addressed most of this above. However, if you want to implement the pixel color issue into play..... you can reduce the MP of the backs by....
30% whens shooting Red and Blue Targets
15% when shooting Green Targets
0% when shooting B&W targets.
Thanks for pointing that out, its rare that anyone will mention that color
targets engineered to test design weaknesses do yield poor results.
Quote:
Originally Posted by
bglick
If you have a digital camera with some good lenses, you can test this by shooting a nice array of color targets, and you will see, after 1/R, these reductions above represent the reality of digital recording of Bayers sensors, using at a min. high end DSLR... we are not comparing a $50 P&S digital camera here. (just to be clear) So a good avg. is 20%... again, we are trying to get in the ballpark in this discussion, i.e. somehow tame 600MP claims down to 40MP... .then one can deal with the nitty gritty details. Of course, these discussions have been going on for years on the internet, most of this is old news.
600MP does need taming (now that the true context of 40MP as been re-stated accurately). From an engineering test perspective I'm uncomfortable assuming
that given the 1/R equation, microscopes sweeping the image/focal/emulsion
plane that there does not exist an optic/film combination lying between
the 40MP and 200MP value.
For strict metrology engineering lab values (nothing the public will see) the 10MP
value and 12bit depth is probably going to stick until repeatable tests demonstrate otherwise.
Futhermore, it is implicit that Industrial and Scientific Tests of the DB should be done raw so that software (an admittedly extremely powerful factor) does not skew highly precise and repeatable measurements.
-
Re: 4x5 Ultra Fine Focusing and Calibration - Cont'd
Continued from previous post (ran out of room) .,.
Quote:
Originally Posted by
bglick
> Admittedly Scheimpflug is out of scope for me. f8-f16 is good for the 35mm or DSLR spotmeter, f11-f22 for 120/220 backs on 4x5, and f32 is good on a sunny day but freezing movment becomes problematic for me.
Scheimpflug has not entered into this discussion, not sure why you mention it.... and as for f stops, I am not following your logic here. A 35mm camera that requries 5.6 for sufficient DOF, will use f11 on MF, f22 on 4x5 and f45 on 810.... maybe what you wrote above was a typo? Simple rule, you double the format size, you double the f stop for the same DOF.
I'm trying to keep the model as simple as possible (my head hurts).
Scheimpflug is way over my head (so thanks for not goingthere).
I do not think Depth of Focus is of any use on small and medium format cameras
because in lies in the emulsion plane (and therfore is zero).
I believe that it is generally accepted principle that with no movements the
depth of focus on a 4x5 can also be zero, which also happens to be the point
where maximum resolution is achieved.
This seems to be disliked by view camera purists. But there are just enough
movements (if you know what you doing) to have a little fun on a press camera
(speed graphic). I know the Toyo is way better but suspect that a 35x loupe
in the image plane will help resolve an airy disk to as close a point as possible.
This could not be done with much accuracy behind Beattie and Fresnel sceens
(with DOF at zero) maybe some of the newer screens would allow this (would
be nice) so all you would need would be the carson 35X pocket microscope
which I think is under thirty dollars now.
Quote:
Originally Posted by
bglick
> Depth of Focus has some value (probably much more to view camera purists), but I have to to resist relying on it as an excuse to not shim the plenum so as to bring Image Focus into the Emulsion Plane.
Yes, both camera manufacturers, film holder manufacturers have done this for us, hence why there is international standards for all these issues. Unless something fell out of alignment, or you are dealing with early 1900's cameras with modern film holders, Depth of Focus at the film plane will resolve any small errors, as suggested in my previous post, by a factor of 15 - 20x, vs. the .001" you are shooting for.
end of part I
...
The only thing I want from Depth of Focus is for it to be zero as to attain the highest
lp/mm measurement possible on a 4x5 with no movements and (if affordable) APO
lenses.
-
Re: 4x5 Ultra Fine Focusing and Calibration
> Is this the same 1/R equation found in the Appendix of this manuscript ?
A more complicated version of this formula, which I have not checked, is found on page 8 of the pdf link you provided. The 1/R equation I used, was explained in detail above.... there is several versions of 1/R, most all producing similar results, at least considering the mega differences this thread is evaluating. Basic 1/R refers to non macro of course.
> If 33lp/mm is all one can get out of 810 then I'd guess (never have used one although they themselves are beautiful functional art) that any kit to help sanity check alignment and perform calibration would be futile if not wasted effort
Or, just simply buy $100 worth of straight edges and dial indicators, and with a little mechanical know-how, you can do simple sanity checks for what you were pursuing.... the more precision you require, the more expensive the test gear. There is a few companies such as Toyo USA that will laser check your cameras, and align them for a reasonsable fee. The problem often is in the film holders, assuming they are not modern ones which were not abused...
> No, I'm NOT extrapolating this to anything larger than 4x5
my point was, you can't even extrapolate up to 4x5.... my previous post explained this in detail....
> To me 4x5 is sort of a a miniature LF and the number value of 68 lp/mm seems within the realm of possibilities (maybe dead on for your high quality film, equipment and experience).
With color film, as I mentioned in my previous two posts, OFR (On Film Resolution) is closer too 27 lp/mm. This assumes 65 lp/mm MTF for the film and f32, just as an example.... the 68 lp/mm you refer to is for lens aerial resolution, i.e. not combined with film, which is where 1/R enters into the equation. The lens aerial resolution measures the MTF value of the optic only. This was just for clarification, as based on what you wrote, I was unsure if this was clear....
> Furthermore, it would seem prudent for little people like my self to sanity check our 1949 vintage gear (yes I rebuilt the focal plane shutter by hand and am proud of it and the history that it represents).
Yep, very valid point. If you do a search on this forum, you will find the dates when international standards came into play... I don't recall... but it was a long ago.... anyway, there is a lot more than gg and film alignment, and film flatness to deal with when checking a vintage camera. The alignment between the two standards is equally (if not more) important. These cameras can take a beating over the years.
> And no (as indicated above) anything less that 70lp/mm is not what I'm generally looking for is high resolution. Any Validation Engineering lp/mm results that do not hold up to a microscope sweeping through the emulsion plane should be called into doubt as a matter of generally accepted good scientific practice.
Agreed, but be sure to check the film plane with a high quality optic, otherwise you will have introduced a new weak link in the chain. A good scaner, such as a drum or high end flat bed is usually good enough, but a microscope is even better, considering you have good vision...
> The Microcope and Stand setup to quantify lens resolution has become very cheap. Anyone can outfit a lab for as little as $3000 (probably a tenth of what some hear drop on thier systems) and verify marketing claims (unless they have a closed black box system). This means the numbers can now be easily verified -
Are you referring to examining film, or aerial resolution of lenses?
> I respectfully disagree with the Foveon citation. The only valid comarison for engineering purposes are the photon-multipliers used in Heidelberg Tango Wet Scans.
Just an FYI.....The Tango Wet scans are by no means a high-end standard today.... there is a host of scanners that supersede what a Tango can do, even high end flat beds, from Creo and Screen. Tango drums were all the rage, but in mid 90's...
> Futhermore, it is implicit that Industrial and Scientific Tests of the DB should be done raw so that software (an admittedly extremely powerful factor) does not skew highly precise and repeatable measurements.
Software is an integral part of the Bayer system. The test results I use, and the ones I cited above all use RAW processing and sharpening. The same is true for scanned film, as this is the situation in the real world, i.e. both images are manipulated to their best possible form. (assuming one is not printing with a darkroom enlarger) Since a Bayer file is 80%+ interpolated data, removing software will cripple its capabilities vs. running it through the software mill, which it was designed for.
> I do not think Depth of Focus is of any use on small and medium format cameras
because in lies in the emulsion plane (and therfore is zero).
I think Zeiss, Leica and Mamiya would strongly disagree with your assertion. They have written numerous white papers through the years stating just how critical film flatness is.... you can find them on the web if you search hard enough. But once again, this is due to their camera systems optimizing lenses in the f1.x - f5.6 range. The Mamiya 7 camera is praised not just for its lenses, but for its extreme film flatness, producing 35mm type resolutions in a MF body.
> This could not be done with much accuracy behind Beattie and Fresnel sceens (with DOF at zero) maybe some of the newer screens would allow this (would be nice) so all you would need would be the carson 35X pocket microscope which I think is under thirty dollars now.
The problem with checking anything with a LF ground glass is the simple fact - a ground glass is VERY low resolving... in the 1 - 6 lp/mm range at best. Glass can be ground finer to resolve better, however then it will transmit too much light through the glass and the image will be loose light intensity, making composure nearly impossible under the hood. It's a trade off..... But this is why anything more than a 5x loupe on a gg is way overkill.... the glass itself the limiting factor. Fresnels complicate the matter even more, as they re direct light.
If you want a high resolving glass screen you should use a 3 micron fiber optic plate which will resolve 150 lp/mm, now you can use a high quality magnifier to inspect the image appearing on the fiber optic plate. I once considered this..... I had a spec. prepared and sent it to a few optical fabricators... for 4x5 size, prices ranged from $5k - $10k and for 810....not worth mentioning. Too rich for my blood... not too mention, it would only be for critical inspection of the point of exact focus, not for composing, as it would transmit the image through the plate, not capture it.
> The only thing I want from Depth of Focus is for it to be zero as to attain the highest lp/mm measurement possible on a 4x5 with no movements and (if affordable) APO lenses.
Depth of focus has no true Zero, UNLESS, you are shooting a flat subject or an infinity subject. This is where film alignment and gg alignment becomes critical, but again, only at wider f stops, say f8 or wider.
When shooting a common scene, such as a landscape, which has depth.....then the Depth of "Field" of the subject side of the lens, say 40 ft to infinity, this translates to a compressed range of focus at the film plane.... in this case, a .07" range, which correlates to 40ft to infinity. This assumes a 150mm fl lens. Where the film plane lies, dictates at what distance from the lens, critical focus is achieved. When you consider all the slop in the mechanics of a LF system, the trueness of the standards, the gearing, the lock downs where slight movements occur, etc. .07" is easy to be consumed, hence why high f stops are almost mandatory in LF....... If you compare the precisioned movements on a high end studio view camera for MF digital backs, vs. a LF field camera, its quite obvious how sloppy things really are.... at least when compared to precision variables we are discussing here.
see next post....
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Re: 4x5 Ultra Fine Focusing and Calibration
Rich, you may find it interesting, ...... what you are trying to accomplish here, (gg / film alignment) is now standard in Canon DSLR's. They now incorporate focus setting offsets you can enter into the cameras memory. After you test for this for a given lens, and make the critical adjustments, every time you use THAT lens, the the plane of sharp focus you see through the viewfinder (our gg), will be perfectly aligned with the digital sensor plane. Such a nice feature, who would have ever imagined this type of technology would be built into consumer grade DSLR's. ???
Of course, with f2 lenses common on these systems, this becomes a very desirable feature. At f2.0, the Depth of Focus at the film plane is about +/- .0015". As you can see, at these LOW f stops, correcting errors at the film plane of .001" is very desirable and EXTREMELY beneficial. But at f32 on LF, the Depth of Focus at +/- .0415", is quite sloppy, about 30 x more sloppy.
There is a good write up on this on the luminous landscape web site.... it also shows a neat product that helps you determine critical focus by taking exposures....this is the same technique I used to confirm my lenses.... this new "lens align" product is ideal for fast and easy digital camera testing of sensor / viewfinder alignment. have a look....
http://tinyurl.com/5pot3p
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Re: 4x5 Ultra Fine Focusing and Calibration
bglick, you described a fiber optic plate above that I am not at all familiar with but sounds pretty interesting. Is this an array of fiber optics (3 um in diameter) all set side by side in an array that could be fabricated in 4X5 format? As such the at focus end of the fiber array could capture the image and transmit it on axis to the rear exit of the array. If the fiber sections are clear that would only produce an aerial image that would need to be intercepted by a loupe focused on the front of the array or the rear of the array? (I think the front, but maybe it does not matter). OTOH the front could be ground finely to produce a real image and the scattered light from that surface would tend to be transmitted on axis back to the loupe. If I'm imagining this correctly the result would be an impressively bright digital screen so to speak. I realize that such a device would be necessarily expensive to produce but sounds as if it would be the cats ass for redirecting off axis rays, especially for wide angle lens applications. I might settle for 10 um fibers at a lower cost (1/10 the number of fibers).
I was interested in the lens align product also - pretty cute. It would appear that a version could be used for LF purposes. The advantage would be that one need not futz with the method discussed above so much but use the device in the center and corners of the film plane for a particular lens. Of course the practicality of the exercise can still be called into question given the plethora of hard to control variables endemic with view cameras.
Another point here as discussed above is DOF. DOF requires one to establish an acceptable Circle Of Confusion prior to strictly defining a Depth Of Focus. Which leads me to another point that I confess I'm fuzzy on. I have always assumed that a point of best focus for an ideal lens is the diameter of an Airy disk and that this point along the image axis is a point that tends to an infinity point. In other words the depth of this point is infinitely small. Any light you can shed on this? It won't help my photography but is intellectually interesting.
BTW this is an interesting post and discussion even tho interlaced with considerable hype. Great thanks for the interaction from both of you.
Nate Potter, Austin TX.
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FYI 4x5 Ultra Fine Focusing and Calibration - Airy Disks
Quote:
Originally Posted by
Nathan Potter
...
I have always assumed that a point of best focus for an ideal lens is the diameter of an Airy disk and that this point along the image axis is a point that tends to an infinity point. In other words the depth of this point is infinitely small. Any light you can shed on this? It won't help my photography but is intellectually interesting.
By Jerry Lodriguss "Astrophotography Techniques"
Ref: astropix.com/HTML/I_ASTROP/FOCUS/DEFS.HTM
Spot Size of the Airy Disk
d = 2.44 x lambda x f -- OR --
d = 0.001586mm x f
Linear Diameter of the Airy Disk (f/22 to f/128 added - see attachment),
I think this is right .,.
Maybe it partially explains why my laser pointer does not look like a perfect point.
Richard
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Re: 4x5 Ultra Fine Focusing and Calibration
> Is this an array of fiber optics (3 um in diameter) all set side by side in an array that could be fabricated in 4X5 format?
This is EXACTLY correct..... Fiber Optic Plates (FOP) can be made with any size fibers, (with 3 micron being about the smallest) and almost any size format, and at any thickness as well. I have made them for different optical products through the years.... however, as they get big, the price becomes exorbitant....as most FOP's are the size of a dime... a big one is the size of a quarter...
> If the fiber sections are clear that would only produce an aerial image that would need to be intercepted by a loupe focused on the front of the array or the rear of the array?
The focus plane is at the front, and is transmitted with virtually no loss in resolution to the rear of the array....where you can use the loupe, its amazingly effective. In the right application, I have used them in between two lenses, and the FOP become s invisible.... but they have 3 micron fibers (1/1000th of a mm). Their purpose is to change the ray angles without sophisticated / expensive optics. They are also used to re direct the location of the plane of sharp focus.
> If I'm imagining this correctly the result would be an impressively bright digital screen so to speak.
Yep, this is true, however, FOP's have an inherent f number, and this is based on several factors.... but like all optics, they eat light, and often this makes their use undesirable when light is limited. But for a gg, this is not such a big deal, at least during daylight hours and say f5.6 lenses.
> I realize that such a device would be necessarily expensive to produce but sounds as if it would be the cats ass for redirecting off axis rays, especially for wide angle lens applications. I might settle for 10 um fibers at a lower cost (1/10 the number of fibers).
You can re direct the rays, and that is part of the beauty of FO in general.... but to accomplish what you desire, I don't think the means will justify the ends. First, the highly angled rays on a WA lens already suffer light loss.... they will loose even more light, maybe up to 2 stops additional. Next, to optimize this process, you would need angled fiber optic strands, to intercept the rays dead-on from the lens, then re direct them to hit the film perpendicular. Net effect IMO is..... loss of light, and marginally better image quality...and such a bundle would be about 5x the price I mentioned above...as this is the edge of technology.
have a look at this pix.... its very close to what you desire.....
http://www.us.schott.com/fiberoptics...ee_400-220.jpg
> I was interested in the lens align product also - pretty cute. It would appear that a version could be used for LF purposes. The advantage would be that one need not futz with the method discussed above so much but use the device in the center and corners of the film plane for a particular lens.
I invented my own version of this 10 years ago, and used it to assure all cameras and film holders were true.... it cost a fortune in film / processing.... I used rear shift and rise / fall to take multiple shots per sheet film to confirm...luckily everything was dead-on, otherwise, I would have had to do this exercise over and over till it was tweaked just right with shims. Later, I used the machinist method for checking everything, much easier, and more reliable, as you say, it eliminates all the other variables. Its best to isolate all the variables when testing. But, with digital, all this feedback is instantaneous. specially when tethered to a pc...... How sweet, huh..... hence why digital is amazing....
> I have always assumed that a point of best focus for an ideal lens is the diameter of an Airy disk and that this point along the image axis is a point that tends to an infinity point. In other words the depth of this point is infinitely small. Any light you can shed on this? It won't help my photography but is intellectually interesting.
I am not sure I am following your question.... but I will take a stab at it... forgive me if I repeat things you already understand. The point of sharpest focus, is where the rays converge to their smallest point, creating the smallest airy disk. Where this point lies, depends on several factors.
Lets take a LF lens for example... as the complicated zoom lenses that have internal focus introduce many variables beyond the scope of this discussion. A single fl lens has one fl, and no single focus point. The reality is, we don't actually focus our view cameras, there is no such thing as "focus" to a lens. Lets assume rear focus cameras for simplification. The lens remains still, it performs no function when you focus the rear. It projects all subjects in front of the lens towards the film. Near subjects converging rays are further behind the lens, vs. infinity subjects. By moving the rear standard up n back (focus) we simply align the gg to intercept the converging rays to produce the sharpness we desire. The wider the apt. of the lens, the greater the converging angles, the smaller the distance of convergence that meets the cc criteria you desire to achieve. This is Depth of Focus. Depth of Field also exist at the film plane .... as all subjects are projecting converging rays at different points in front of and behind the film plane.
Thats the basics....but how small the convergence becomes (airy disc diameter) depends on many other factors, unfortunately. But the ceiling is set by apt. diffraction, which is why see this mentioned so often, but this is simply a ceiling, not a true value for all lenses. A LF lens not only has ONE fl that never changes, it also has ONE focus distance it is optimized for, where the airy discs will be the smallest, and also ONE or maybe two F stops where the airy discs are closest to the limits of apt. diffraction. The reason for this is.... all lenses are designed with certain input criteria, such as subject plane distance, image plane distance, stop diameters (apt.), image circle size, etc. This is the basis of the lens design.... after this is accomplished, the designer evaluates how bad the image becomes when you wander from the "design conditions". This determines if a lens design is suitable for a given task. As a general rule, the better a lens performs at the design condition, the worst it will perform at non-design conditions. This is why we have makro lenses, as a lens can not be optimized for infinity AND close focus. This is also why lenses should be shot at their design f stops...or higher, as there often is little or no penalty for raising the f stop, as apt. diffraction becomes the limiting resolution factor.
So the point is, the smallest airy disk a lens can produce is not cut n dry... as a general rule, it will exist at the design subject distance, at the design f stop, and often closer to on-axis vs. off axis. This subject distance could mean infinity, or 6" from the lens, or at f5.6 or f32.... this is what MTF charts tell us....unfortunately, IMO, they don't give us enough charts, but some are better than none... Schneider is getting better at this, Rodenstock still far behind.
Of course, then you have to hope the lens was built to spec and tolerances were held, then tested to confirm aerial resolution. I know this is the case with the new digitar lenses. I don't think this is the case with LF lenses.... hence the bad batches you hear about, such as the SSXL 80's several years ago. There reaches a point where makers just gamble on QC, otherwise lens prices can start doubling very fast.... I think LF lenses are a bargain, all things considered.... Today though, computers have sped up the lens QC procedures, within a few minutes a lens can be analyzed for MTF at different focus distances and f stops, distortion plots, image circle diameters, back focus distance, coma, astig., etc. etc. Most all the big makers today use this equipment. Prices have fallen dramatically in the past 5 years.
There is one other caveat.... everything mentioned above assumes spherical elements...the norm up to 5 - 10 years ago. Aspherical elements change all the rules. Their biggest benefit in camera lenses is their ability to level resolution for on axis vs. off axis rays. In some ashperical element lenses, we can see sharper edges than we see in the corners. This is often desirable, as the center area of an image is only a small % of the total image area. Anyway, just wanted to mention aspherics, as they are playing a big role in modern photo optics. Is anyone aware of LF lenses with ashperics elements being used? I think I recall seeing a few advertised?
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Re: 4x5 Ultra Fine Focusing and Calibration
bglick, many thanks. I think you inadvertently answered my question which referred to an Airy disk. I was not very precise in formulating the question.
I clearly get your point about light loss when trying to use a Fiber Optic Plate to transmit strongly off axis rays; DUH. One needs to capture a ray orthogonally to the face of the fiber to achieve maximum transmission.
I was not after the diameter of the Airy disk formed by a lens of a particular NA but wondering more mathematically about the length along the optical axis (that is if you will its equivalent depth of focus) for an object which is in a single plane and for, ideally, using monochromatic light. Thus setting the conditions of a single object plane and monochromatic light the depth of the Airy disc along the optical axis will be about infinitely small. Of course using the whole visible spectrum red and blue will fall slightly in front of and slightly in back of the plane of best focus for green depending on the degree of color correction of the lens. And also objects further behind and closer to the lens will be imaged sharply in front of and behind the plane of best focus chosen that I mentioned above. All this you have alluded to above.
I think rvhalejr answered my same question thinking that I was referring to the diameter of the Airy disk. Sorry to mislead.
The only LF lenses which employ aspheric elements I am aware of are the Super Symmar aspheric series I think introduced about 2000. There is the 80, 110, 150 and 210 at least and maybe a couple of other exotics. And indeed I believe the aspheric element or maybe elements are used to improve the far off axis resolution.
Nate Potter, Austin TX.
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Re: 4x5 Fing Focusing - Airy Disks and Depth of Focus
I've never really studied Depth-of-Focus and what happens to an Airy Disk on a
4x5 system. It does seem to be a big issue now (DOF - not Depth of Field), so
I'm going to measure it when time permits.
Preferred Simple tools are clear glass, 35x Loupe and the 100x with reticule (when there seem to be commonly held opposing views about, well, the validity of anything and everything).
In the off chance that anyone has used a device (like sinar image attached) to
gather lens lp/mm data (about the point of best measured focus) or has seen it published I would like to study it.
The characterization of what the perfect point does in the depth of focus envelope
(observable measurements) help formulate a clear picture from which generalizations might be drawn. I can get data (myself) for incremental changes along the the film plane's normal vector and extrapolate, but that may be to prone to error (and maybe not even reproducible).
I'd like to see it for an intentionally angled rear standard[1] which is is moved in incremental steps along the the film plane's normal vector.
In LF Photography this may follow the "there is no perfect focus" technique by advocated by the pinhole police[2]
What bothers me about the rich and famous here is that they never have the time
to post a picture of flat unexposed film in a film holder with a reflection off the
emulsion that is not distorted.
They have access to MILLIONS of stock photos but just cannot provide such
a simple image, "OH ALL MY FILM IS PERFECTLY FLAT", well fine, then please
show us.
Well leave the "OH IT DOES NOT MATTER BECAUSE OF THE MAGIC Depth
of Focus ENVELOPE" until there is some repeatable direct measurement data
has to what happens to the airy disk in there.
Please try to restrain yourself from condescending remarks. If you disagree then
please do so respectfully[3]. Broad brush characterizations like "hype"[4], etc.
when someone is trying to conduct a simple impartial engineering validation.
Good etiquette need not be ignored[4], even if speculation is considered to be
over the top. There is no top in science as long as step by step directly
measured repeatable data is available to validate a reasonable limit.
For so many to resist (if not hate) the idea of putting magnifying devices in the emulsion plane and then making lp/mm measurements at f-stops known to be the
sharpest speaks volumes.
---------------
Notes
[1] ref en.wikipedia.org/wiki/View_camera
[2] No picture under f64 is an LF picture
[3] This is a "one star thread" by definition the worst of the worst.
[4] I'm ripping up my degree in digital electronics, swallowing it and
eventually depositing it in the same place any ideas that do not
conform to those of the mega pixel police get relegated to.
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Re: Ultra Fine Focusing Image and pdf file summary
Ultra Fine Focusing Image and pdf file summary
Posted 1/1/09
1.) Page 2 of 12 - "4x5 Fine Focusing Loupe and Precision Plenum Photography" What happens to a resolution chart with a variance of +/- .012" as compared to and "film plane in optimum position" annotation (a Sinar source)
2.) Table of Linear Diameters of the Airy Disk (f/22 to f/128 added)
Posted 12/24/08
1.) Page 1 of 18 4x5 - "Film Holder and Flatness Metrology" showing unflat film out of the box and distorted reflection of emulsion when placed in typical
wooded film holder (critical dimension .192 - .193).
2.) What a mirror like flat reflected image looks like (rolled and taped to glass plenum in the same way an Heidelberg Tango fluid mount is).
3.) Page 1 thru 4 "The Standoffs for Off-Center Focusing" showing 35x loupe on shimmed plenum with line-of-sight stands for center, middle and corner of frame.
Engineering a Simple Validation Test:
This is setting the ground work for attaining the best possible focus onto
the flattest possible film in a calibrated film holder using resources common
to everyone.
Constraints:
Avoid extreme apertures known to have detrimental
effects on lp/mm measurement.
Whats the hold up ?
Need a source of out-of-the box flat film to test.
A non-distorted mirror image from the reflection on the emulsion out of
the box is needed. If anyone is well connected enough to obtain such
film please contact me.
The non-distorted mirror image is an indication the film is probably
flat to +/- .001" or better.
More time should be spent making bracketed test shots and test time
instead of trying to defend what is logically the next step after repeatedly
snapping in focus across the entire 4x5 frame using aligned glass and
35x loupe.
This should be a fairly boring confirmation of what is already known.
Many seem ready to reject results and attack the validity of the test
if the results conflict with their beliefs.
I plead guilty expressing unrealistic hopes and expectations in some instances. Its based on a big investment in a "Universal upgrade to CCD fluid scanning systems" Should I have to reveal every detail in order for the reader to allow for the possibility ? Off course not. At least not until I can get some really flat focus on some really flat film and determine what the limits are. For all the skeptics know I may be serious about ISO 25, 12 and maybe 6 film if its already flat. Please allow for the possibility that a fresh set of eyes might find something new.
But that does rise to the level or justify disregarding out-of-hand results (before the tests are even started) because they MAY NOT fit the fervently held canonical belief that only two fstops than can be used for Large Format Photography.
The only unbiased conclusion that can be drawn beforehand is that there MAY be an alternative range that could be exploited depending on ones style or desires.
I just want to get the sharpest image possible. Even if the data shows that other f-stops, other than those convention holds we should use, are more precise with respect to lp/mm targets (ariel modality) does not mean that the conventional fstops are deprecated in any way.
So if anyone wants to help out please recommend a source of film known to be flat.
And if you want to criticize then feel free, but you are expected to support your opinion in the same context as this thread. If you have flat film then post a picture.
If your magic DOF (Depth of Focus) justifies something (fstops, non-ariel focus, etc.) then post images, sketches or cite references that illustrate your claim. The use of
DOF to describe Depth of Focus and/or Depth of Field is obfuscation at its worse for
some of us who do not have "i know the context" built into our DNA especially when
they get intermixed (sigh).
I feel like an idiot for even making two 8x10 plenums and prototyping the corner stand. I honestly wondered if AA used this technique and did not tell anyone. I'm fairly sure that he at least experimented with glass plates and new the value of flatness. I'm no AA expert but Ive been lucky enough to have seen his work up really close and read a few of his books.
On any given day Yosemite is but 4 or 5 hours away. But any dream of going there with an 8x10 has pretty much been crushed. I'm still holding out hope that the 4x5 might work.
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Re: 4x5 Ultra Fine Focusing and Calibration
Rich, I have been very respectful of your posts, and am only trying to offer input based on my experience. If you find any of this offensive, I will immediately stop. On these forums, its difficult to tell when someone takes offense to others opinions. I can assure you Nate meant no harm in his remarks, he has been an avid supporter of this forum and its members for many years.
> For so many to resist (if not hate) the idea of putting magnifying devices in the emulsion plane and then making lp/mm measurements at f-stops known to be the sharpest speaks volumes.
I probably should have expanded on this, as my point was not well received. There is a few different issues at play here:
1) The issue regarding film holder flatness is two fold. First, the flatness of the film holder plate, where the film lies. This needs to be flat to the film holders border, where it makes contact with the view camera back. Second, the view camera back, where it makes contact with the film holder, must also check flat. The addition of a flat granite surface makes this part easy to test. My position is still the same, this is best performed with off-the-shelf straight edges with a .001 or, as high as .0001" dial indicator. The former is more than adequate and can be had for $15 - $100 based on the brand name.
2) Checking resolution at the film plane is a different issue. This is referred to as aerial resolution, AND it can only be checked accurately by another set of optics at the plane. (or it can be reverse calculated with film, more on that below) The most accurate method would be too send the lens off to a optics testing lab, they can give you an entire report on MTF at any f stop, any back focus distance you spec, and MTF data for designated points on the image circle radius. It would be nice if manufacturers provided this info with each lens they produce, but then, it would create "cherry picking" as everyone would want the primo lenses, not the ones which just barely passed the min. performance thresholds. My guess is, these tests would run $150 - $200 per lens....
A home-brew method of doing the same can be accomplished....but with certain caveats. The subject side of the lens should have targets placed at the desired distances. Image magnification must be determined to know which set of targets equate to lp/mm at the film plane. This is best accomplished by using one large target, and measuring it on the gg, now the image scale is set, which you can apply to the test targets.
Now, the hard parts..... assuming you want to use a loupe, the key is to provide a flat surface the loupe can ride on, but be true to the image plane. I once considered doing this.... and my idea was, use a piece of black FLAT acrylic plastic, (low cost and probably flat to .002" at 4x5 size) and draw 4 radius lines from the center outward to each corner....then drill 4 holes on each radius, allow the loupe to view through these holes....you can now take readings at each radius point and record them.
The loupe and the eye are the next obstacles. Lets assume you have good vision and can resolve one arc minute. (mostly younger people < 45 years of age with no vision problems) If you are testing a lens at f8 the max. aerial resolution delivered to the film plane is limited by apt. diffraction at about 188 lp/mm. This is an avg., you can work out the details for all the different wavelengths of light....this is a general discussion to execute the test so the results make sense, as often is the case, the test results demonstrate other weak links in the chain, vs. the variable you were trying to test...... so the tester is fooled.
So what fl loupe is required to resolve 188 lp/mm? There is where many variables come into play. We will work with well accepted conventions here. First, the eyes resolving capability is inversely proportional to viewing distance, i.e. as we double the viewing distance the eye resolves half, and when we half the viewing distance we can resolve 2x. The conventional viewing distance is 250mm / 10". At this distance, here is a well accepted range of resolution of human vision....
2 lp/mm - low end of vision, seniors, low fovea resolution, abberated eye lens, large astigmatisms, etc.
5 lp/mm - VERY good human vision, often <45 years of age, no vision problems.
10 lp/mm - Excellent human vision, often <30 years of age, low abberations in eye lens, high density fovea. (you qualify for Top Gun fighter school)
If a person falls into the first scenario, at 2 lp/mm resolving power at 250mm viewing distance, we must reverse calc. at what viewing distance his eye must be from the image plane to resolve 188 lp/mm. 188/2 = 94. We must reduce the 250mm view distance by 94%, or 6% * 250mm = 15mm. Therefore the eye must be placed 15mm from the image plane for the observer to resolve a 188 lp/mm target at the image plane. So therefore a 15mm magnifier or film loupe is required, at a min. The quality of the loupe comes into play next. If you use a high end loupe, you can often acheive 80%+ MTF value, which is good....however, lower end loupes, can barely hit 50% MTF, making the loupe the limiting factor. In which case, you should halve the fl of the loupe provide better resolving capability (double magnification), in lieu of higher MTF loupes (more expensive) . Lets consider a 7mm fl loupe, which is 36x. (250/7 = 36x) So at f8, your carson loupe is at its limit for this test, and this assumes it has good enough MTF so it does NOT become the limiting factor to test. You can step up to a good astronomy Eye Piece at 7mm fl for about $75 - $100... since you only use the loupe on-axis, any good plossl design EP in the 7mm fl range will suffice.
Next, the eye... a big part of the equation. The MTF and resolution of the eye is HIGHLY dependent on the pupil diameter..... at low pupils ~ 2.5mm the eyes MTF curve (the eye is a lens, it also has MTF curves), shows about 70% MTF, not too bad. But at 4mm diam pupils, the MTF falls to below 25%, very poor, in which case, the eye will be the limiting factor. In other words, to put the eye in "inspection mode" and not allow the eye to be the weak link the optical train, your light source on the target must be ultra bright.... at a bear min. direct sunlight on the target, maybe more, which is dependent on the fstop you are testing the lens at. You need about 15EV to dilate the pupil down to 2.5mm.... you can easily check this at the image plane with a light meter. Anything less than 15EV, your eye will limit what you can resolve and give false readings...
Next, you need the loupe to be on-axis with the lens rays, so a tilting loupe is required.... you may need to jury rig something for this, or have an ultra steady hand... again, another variable... A tilting field microscope would be ideal.
or....... the simpler alternative, which is easier, cheaper and more accurate is....
Once you confirm the camera system is square, flat and gg / film plane are in alignment, you can shoot a piece of film at each f stop, and evaluate the results on a light box or even better, a cheap microscope. Since we know the MTF value of the film at a given contrast (targets), you can simply use 1/R and reverse engineer to find the lens aerial resolutions at each f stop. Thats the beauty of 1/R, the only missing variable in the equation is the aerial resolution of the lens. Careful film processing should be assured to assure the captured resolution is displayed in the processed film. (not a huge variable)
To be sure the film is laying flat, you may consider tilting the camera up a bit, so gravity pulls the film flat. You will have to place the targets higher of course. Of course start with flat film fresh out of the box. This is where a simple vacuum back would be nice.
Both methods will work, but the film version has a LOT less variables to control, hence why it will produce reliable and consistent results as a home brew testing method.
Your next post.... Part II.....
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Re: 4x5 Ultra Fine Focusing and Calibration
> The non-distorted mirror image is an indication the film is probably flat to +/- .001" or better.
Rich, can you describe what procedure you plan to use to see a perfect mirror image on film? do you shine light on it? BTW, mirrors are way flatter than .001".... but I am confused how you are implementing this mirror technique to check if the film itself is flat???
> after repeatedly snapping in focus across the entire 4x5 frame using aligned glass and
35x loupe.
I mentioned this in previous posts, I don't know if you disagree with me, or you did not read it....but there is NO glass that can resolve the detail the lens will project. Not even close... you are comparing a glass that might resolve 5 lp/mm vs. a lens that is projecting 100 - 300 lp/mm ? The weak link is the first part of your optical chain, agree? My last post explains how you must intercept the rays directly with a second optic right at the image plane, without the aligned glass. Maybe after you read this, you can comment....
> I plead guilty expressing unrealistic hopes and expectations in some instances. Its based on a big investment in a "Universal upgrade to CCD fluid scanning systems"
Unrealistic expectations are what these forums are for :-) Often, there is reality hackers out there, that are ready to help, and....for free....not so bad IMO... What CCD fluid scanning do you refer to? I am currently use a Screen Cezanne Elite which works very well wet mounted up to 5300 dpi... previously, I owned several drums scanners... I find these high flat beds as good as the best drums. The one exception may be the ICG drum scanners, which I have never used... but my guess is, any improvements would be marginal, in the 5% range for both d range and resolution.
> At least not until I can get some really flat focus on some really flat film and determine what the limits are.
I can assure, you, these limits have been well defined for many years. The only addition that testing offers is, just how close your equipment can be to these known limits. Exceeding the limits, it's not in the cards, hence why all this testing, at best, will give you an estimate if you are "at" the limit, or maybe 30% from the limit..... not a huge range, for the amount of work involved. This assumes decent gear, relatively modern lenses, etc. However, the complexity of the tests and all the variables involved can often lead people to believe they re-wrote every optical principle known to mankind :-)
> MAY NOT fit the fervently held canonical belief that only two fstops than can be used for Large Format Photography.
I will assume you are commenting on my previous post. I never stated that only two f stops can be used.... please re read. What I stated was, typically there is ONLY two f stops on LF lenses where the MTF is optimized. You can see this the MTF curves the manufacturers provide. The fall off is dramatic. You can use any f stop you desire, assuming you are not concerned with achieving the max. image quality that lens can is capable of delivering.
Also, remember, its the contrast that is delivered to the film that matters here, not the resolution. Human vision is more sensitive to contrast than pure resolution, specially in the 2 - 5 lp/mm range. So regardless of what is "resolvable", this does not represent the full story behind image quality, hence why MTF has replaced "resolution" as the standard for expressing image quality.... about the mid 1970's, MTF became the standard to express image quality, not resolution, although the two are obviously related. One look at Mamiya 7 color chromes on a light box will make this point ultra clear - a picture is worth a thousand words...
> So if anyone wants to help out please recommend a source of film known to be flat.
How are you confirming all this film is so un-flat? I can't believe I have been this lucky for the past 15 years? I just layed a piece of 8x10 unprocessed Velvia on a Starett 18" square granite stone, certified to +/- .001" flat across its entire surface .... the film lays DEAD flat. In addition, when i insert the film in my Toyo 810 holders, the edges of the film holders are rather taught, stopping the film from buckling from gravity. Can you better explain the problems you are having? And how you determine these problems?
> And if you want to criticize then feel free, but you are expected to support your opinion in the same context as this thread. If you have flat film then post a picture.
I hope you understand, no one is criticizing you. Post a picture of what? Flat film laying a on granite stone? If the film was out .002", could you see this on a picture posted to the web? how? My guess is, you are using this mirror concept, thinking you need 1/8th wave flatness for the film to flat, this is NOT the case.... but maybe I am not understanding you.... so i await...
> The use of DOF to describe Depth of Focus and/or Depth of Field is obfuscation at its worse for some of us who do not have "i know the context" built into our DNA especially when they get intermixed (sigh).
Rich, these forums are for people to share ideas and to help each other. In this case, I have pointed you in the right direction, and spent too much time already on these posts, as these subjects are not easy to explain clearly in a few sentences. However, I can not spend days putting together tutorials to teach these subjects in detail. Read some photo optics books, check Amazon, they have MANY of them, all for different levels....they contain the graphics, math, and explanations that you need to fully understand the principles which you are addressing in this thread. I can assure you, once you have a better understanding of the optics, your positions on these subject matters will change.
> I feel like an idiot for even making two 8x10 plenums and prototyping the corner stand. I honestly wondered if AA used this technique and did not tell anyone. I'm fairly sure that he at least experimented with glass plates and new the value of flatness.
You should never feel like an idiot for experimenting with any of this Rich, I totally respect your tenacity... which is part of the reason I have worked so hard to explain all this in such detail.. it's all part of the learning experience.
As for AA.... I sincerely doubt he did any of what you suggest, mainly because, it simply is not necessary. What AA understood well was that long fl lens (required on 810 format) had very little DOF....or when high DOF was used, it would dramaticaly reduce resolution. (DOF = Depth of Field, Depth of Focus is spelled out to prevent confusion). So to max. OFR, he understood it was important to push the near subjects far from the lens. This was a secret style he kept hush for years, i think he started it in the late 50's or 60's.... he used to mount his 8x10 on 15 ft high ladders to push the nears out, so the image would produce tremendous sharpness at f32. In his later years, he shot atop his motorhome. I copied this method for many of my 810 shots, its very effective, as you are almost taking an infinity shot, which maximizes the recorded resolution, as you can often shoot at f22 or 32 vs. f64. Its this resolution, whereas the big format differentiates from the smaller formats. It all comes down to a better understandings of the optics math. OFR is a function of aerial resolution which is dictated by apt. diffraction, combined with the MTF of the film used. Since AA shot almost all B&W in 810, it's no coincidence (or magic) his images are deemed utlra sharp, even by todays standards, specially considering the small prints he often makes. In my world, I print up to 10 ft long, so I consider AA prints small, its all relative.
Of course, many on this forum are just as capable of producing the same (or better) today, specially due to better lenses and better films, and much more precise printing methods that offer better tonality, digital sharpening, larger color gamuts, etc. etc. The point being, AA's success was not due to ultra high precision gg / film alignment - to better than +/- .001". Depth of Focus took care of this.... Instead, he learned the value of lower f stops to maximize OFR, which is why many of his shots do not have near subjects.
.
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Re: 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
Peter J. De Smidt
Not that long ago, I exchanged info on line with a guy who made a DIY 4x5 vacuum holder. It didn't look that hard to do. It turns out that a rubber bulb pipette bulb has enough suction. I no longer have the contact info, but a little Googling should turn it up.
I really want to get the 4x5 sharpness I'm seeing onto film. Thats with a Depth of Field of 10ft, 40ft and 300ft.
Its with the 4x5 clear plenum calibrated to a .190" critical dimension at f22 with a
Schneider-Kreuznach Angulon 1:68/90 S/N 5415433 restored Hollywood ready 1949
Speed Graphic. Whats even more of a surprise is that its holding almost the same sharpness into the corners, this from the most inexpensive WA German lenses I
have and my budget is been about zero for the last year.
Anyway, to remove flatness as a variable I'm going to have to try the V-Back again. All I need to see are holes that do not rumple the film as its sucked flat over the hole. I'm going to try .050" and then maybe .020" but I may not be able to get much smaller than that.
As the diameter gets smaller the number of holes along the edge may need to increase. My last attempt at .125" and 100 holes per holder failed (as mentioned
before due to rumpling or distorting the image). I'm hoping that the density on the smaller holes will not go up much as it would take forever to drill and hone 200 holes, 400 would be a month long project.
My roll and tape method progressed but I've seen nothing of the USB IR camera on
order and do not have the time to fuss with one trying in hopes of removing the IR filter (to roll and tape unexposed film in darkness). Then there is the matter of routing a precision .030" channel in the holder for the bigger septum to slip into.
I do have a set of four holders (eight septums at .192 or .193) but have not
been able to find film that is +/- .001 out of the box to match (not that it even
ever existed except for glass plate). I've posted pictures of out-of-the box
curled film and the distorted image from the refection off the un-exposed
4x5 velvia emulsion, outside with full sun in my face and captured by an old Nikon DSLR f8 1/25 ISO 2000 with gamma amplified in photoshop to about 4 so as
everyone can see the mirror like distorted reflection.
I also shot a rolled and taped mounted (perfectly flat like is done for a fluid
drum scan) with a perfect mirror like reflection.
The Proof for Mirror Like Reflection Flatness
Skip this unless you really got to know
I've been using the mirror like reflection as a standard of near-perfect flatness,
no tools or measurement needed if the following is true. Window Glass is usually flat to .001" so it works fine. If I stick a .0015 feeler gauge shim behind the film and roll over it I can see the distortion in the reflection on the film. If I stick a finger-nail flattened piece of common household aluminum foil underneath the film and roll over it I can really can not make out any distortion in the film emulsion reflection with the unaided eye. Since Flattened Aluminum foil is consistently .0005" and the smallest automotive feeler gauge .0015" it is likely that film flatness is between the two values, .001" which is (for my purposes) perfectly flat in a carefully engineered
metrology context (or at least reasonable approximation). It also helps that
whichever way I measure my batch of velvia it comes out to be .0095" thick.
End of really detailed information
All the photographer needs to know is if you can get a mirror like reflection
(no distortion) off the emulsion of a piece of film then its plenty flat.
I've got an excess of out-of-the-box curled junk so if I bought more I'd want to send it back if it was curled as well (and no-one will want to do that).
All the Best,
Richard
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Re: 4x5 Ultra Fine Focusing and Calibration
Quote:
Originally Posted by
bglick
Rich, I have been very respectful of your posts, and am only trying to offer input based on my experience. If you find any of this offensive, I will immediately stop. On these forums, its difficult to tell when someone takes offense to others opinions. I can assure you Nate meant no harm in his remarks, he has been an avid supporter of this forum and its members for many years.
The value of what you all are writings is worth far more than what I am able to contribute. My mission (to obtain the best results from what often turns out to be marginal equipment) would be better served in another thread created for the purpose of a structured discussion and planning, procedural documentation, execution and analysis of results. Predictive models and experience are useful, but not at the expense of validation testing. My goal is to obtain the best possible image using tools that make up a simple kit. The 35x Loupe may not be ideal but even it scares a lot of photographers and is far more powerful than the conventional (up to 10x maybe). The 100x with reticule is overkill, but tends to settle a good deal uncertainty when needed. The 400x microscope will remain in the closet as I took and published my last set of micro photographs (of racking through emulsion layers) a long time ago.
Quote:
Originally Posted by
bglick
> For so many to resist (if not hate) the idea of putting magnifying devices in the emulsion plane and then making lp/mm measurements at f-stops known to be the sharpest speaks volumes.
I probably should have expanded on this, as my point was not well received. There is a few different issues at play here:
1) The issue regarding film holder flatness is two fold. First, the flatness of the film holder plate, where the film lies. This needs to be flat to the film holders border, where it makes contact with the view camera back. Second, the view camera back, where it makes contact with the film holder, must also check flat. The addition of a flat granite surface makes this part easy to test. My position is still the same, this is best performed with off-the-shelf straight edges with a .001 or, as high as .0001" dial indicator. The former is more than adequate and can be had for $15 - $100 based on the brand name.
My wife would kill me if she saw the surface plate. An off-the-shelf straight edge
is part of the kit, as are glass gauge blocks, a feeler gauge and glass plenum (that can substitute as a surface plate as its flat to within about .001" . As an option Digital Calipers can be added, honed and with the .0005" touch feature.
From a metrology perspective your setup is superior to mine (in a lab calibration
context) but I do have a lot of high precision machine shop experience (prior to
going back to school) and believe we both have identified the important datums
that most mechanical engineers would want us to pay attention to.
I have some sketches and pictures of this process but might need to add more
to be sure all the bases are covered. From what I've seen the Stainless Steal
Straight edge, gauge block and feeler gauge are accurate enough. Admittedly
the dial indicator on the gauge block is easier than a feeler gauge. But my wife
would divorce me as as the dial indicator and surface plate would make my garage
into a complete machine shop (and she does know what one looks like).
It would not hurt to compare both (I would need to be really stealthy about it)
but go-nogo gauging and .0005" touch are probably good enough for this old
inspector.
Quote:
Originally Posted by
bglick
2) Checking resolution at the film plane is a different issue. This is referred to as aerial resolution, AND it can only be checked accurately by another set of optics at the plane. (or it can be reverse calculated with film, more on that below) The most accurate method would be too send the lens off to a optics testing lab, they can give you an entire report on MTF at any f stop, any back focus distance you spec, and MTF data for designated points on the image circle radius. It would be nice if manufacturers provided this info with each lens they produce, but then, it would create "cherry picking" as everyone would want the primo lenses, not the ones which just barely passed the min. performance thresholds. My guess is, these tests would run $150 - $200 per lens....
A home-brew method of doing the same can be accomplished....but with certain caveats. The subject side of the lens should have targets placed at the desired distances. Image magnification must be determined to know which set of targets equate to lp/mm at the film plane. This is best accomplished by using one large target, and measuring it on the gg, now the image scale is set, which you can apply to the test targets.
Now, the hard parts..... assuming you want to use a loupe, the key is to provide a flat surface the loupe can ride on, but be true to the image plane. I once considered doing this.... and my idea was, use a piece of black FLAT acrylic plastic, (low cost and probably flat to .002" at 4x5 size) and draw 4 radius lines from the center outward to each corner....then drill 4 holes on each radius, allow the loupe to view through these holes....you can now take readings at each radius point and record them.
The loupe and the eye are the next obstacles. Lets assume you have good vision and can resolve one arc minute. (mostly younger people < 45 years of age with no vision problems) If you are testing a lens at f8 the max. aerial resolution delivered to the film plane is limited by apt. diffraction at about 188 lp/mm. This is an avg., you can work out the details for all the different wavelengths of light....this is a general discussion to execute the test so the results make sense, as often is the case, the test results demonstrate other weak links in the chain, vs. the variable you were trying to test...... so the tester is fooled.
So what fl loupe is required to resolve 188 lp/mm? There is where many variables come into play. We will work with well accepted conventions here. First, the eyes resolving capability is inversely proportional to viewing distance, i.e. as we double the viewing distance the eye resolves half, and when we half the viewing distance we can resolve 2x. The conventional viewing distance is 250mm / 10". At this distance, here is a well accepted range of resolution of human vision....
2 lp/mm - low end of vision, seniors, low fovea resolution, abberated eye lens, large astigmatisms, etc.
5 lp/mm - VERY good human vision, often <45 years of age, no vision problems.
10 lp/mm - Excellent human vision, often <30 years of age, low abberations in eye lens, high density fovea. (you qualify for Top Gun fighter school)
If a person falls into the first scenario, at 2 lp/mm resolving power at 250mm viewing distance, we must reverse calc. at what viewing distance his eye must be from the image plane to resolve 188 lp/mm. 188/2 = 94. We must reduce the 250mm view distance by 94%, or 6% * 250mm = 15mm. Therefore the eye must be placed 15mm from the image plane for the observer to resolve a 188 lp/mm target at the image plane. So therefore a 15mm magnifier or film loupe is required, at a min. The quality of the loupe comes into play next. If you use a high end loupe, you can often acheive 80%+ MTF value, which is good....however, lower end loupes, can barely hit 50% MTF, making the loupe the limiting factor. In which case, you should halve the fl of the loupe provide better resolving capability (double magnification), in lieu of higher MTF loupes (more expensive) . Lets consider a 7mm fl loupe, which is 36x. (250/7 = 36x) So at f8, your carson loupe is at its limit for this test, and this assumes it has good enough MTF so it does NOT become the limiting factor to test. You can step up to a good astronomy Eye Piece at 7mm fl for about $75 - $100... since you only use the loupe on-axis, any good plossl design EP in the 7mm fl range will suffice.
Next, the eye... a big part of the equation. The MTF and resolution of the eye is HIGHLY dependent on the pupil diameter..... at low pupils ~ 2.5mm the eyes MTF curve (the eye is a lens, it also has MTF curves), shows about 70% MTF, not too bad. But at 4mm diam pupils, the MTF falls to below 25%, very poor, in which case, the eye will be the limiting factor. In other words, to put the eye in "inspection mode" and not allow the eye to be the weak link the optical train, your light source on the target must be ultra bright.... at a bear min. direct sunlight on the target, maybe more, which is dependent on the fstop you are testing the lens at. You need about 15EV to dilate the pupil down to 2.5mm.... you can easily check this at the image plane with a light meter. Anything less than 15EV, your eye will limit what you can resolve and give false readings...
Next, you need the loupe to be on-axis with the lens rays, so a tilting loupe is required.... you may need to jury rig something for this, or have an ultra steady hand... again, another variable...
A tilting field microscope would be ideal.
Continued in subsequent post (we ran out of room).
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Re: 4x5 Ultra Fine Focusing and Calibration
Continued from previous post
I've fabricated and tested the angled stands for the 35X and they cover the
entire 4x5 frame. Could probably come up with something similar for the 100x.
Take a little practice but its very solid, no rock, but it ain't easy, it can blow
up during machining and about half get tossed (50% yield) There a a lot of
angles to those things and a good deal of honing so that there is no rocking.
Quote:
Originally Posted by
bglick
or....... the simpler alternative, which is easier, cheaper and more accurate is....
Once you confirm the camera system is square, flat and gg / film plane are in alignment, you can shoot a piece of film at each f stop, and evaluate the results on a light box or even better, a cheap microscope. Since we know the MTF value of the film at a given contrast (targets), you can simply use 1/R and reverse engineer to find the lens aerial resolutions at each f stop. Thats the beauty of 1/R, the only missing variable in the equation is the aerial resolution of the lens. Careful film processing should be assured to assure the captured resolution is displayed in the processed film. (not a huge variable)
To be sure the film is laying flat, you may consider tilting the camera up a bit, so gravity pulls the film flat. You will have to place the targets higher of course. Of course start with flat film fresh out of the box.
This is where a simple vacuum back would be nice.
I've resigned my self to trying that again (ugh).
A regular holder can work if the film is not curled. But unless some turns up that looks
like a mirror (turn on an overhead photo light and sweep the reflected emulsion image
back and forth until the entire piece of film as been examined, if it is all mirror like its
flat, if there is distortion it is not). Deforming the film with .0005" shim and .0015"
feeler gauge tells us the the lowest level of out-of-flat distortion is between the
two at .001" which for our first approximation should be acceptabe.
Quote:
Originally Posted by
bglick
Both methods will work, but the film version has a LOT less variables to control, hence why it will produce reliable and consistent results as a home brew testing method.
Your next post.... Part II.....