4x5 Ultra Fine Focusing and Calibration

[rvhalejr]

> ...
but my comments have fallen on deaf ears...

Quite the opposite. Every word you have written, every thought expressed will
be carefully analyzed, researched and studied. Your kindness in reeling me
back in is to be emulated and admired.

I just can't turn on a dime, old people get to move slow, so there you go.
Sometime the wisest thing one can do is not make any comment at all,
which I admit to be guilty of.

I apologize about the once every couple of years thing. What a pain.

I will try to be the best student of the optics discipline I can be. Obviously
there is a lot to catch up on.

I think Your wonderful. If I ever find that your speaking out here I'll be the guy
who falls to your feet and kisses your shoes, and be proud of it.

[rvhalejr]

Does any one have a source for really flat 4x5 ?

Out-of-the-box Really flat film test:

1.) Place on a glass table top, gently tap the emulsion surface with
a ball point pin. (Hint: The reflection off the emulsion should not change
where the tip of the pin touches the film).

2.) The reflection off the emulsion under bright light should look like a
typical hand mirror (no circus mirror distortion around the edges).

3.) For the mechanically inclined, a .0015" feeler gauge should not
be able to slip in between the glass table top and film. Flattened
household aluminum foil is ok (it runs between .0005 and .001")

Please Note:
I do not mean to antagonize or disrespect that part of
the LF community who feel this is a waste of time.

FYI: My Reasons are as follows (if you are curious):
A.) I would like to keep the emulsion plane as flat as possible during
exposure (and of course fluid scanning) and save the the Depth of
Focus envelope for artistic expression, stylistic preference or just
flat out cover up my mistakes.

B.) I am old, my eyesight sucks and "c" is about the size of a dime
or .2 lp/mm (at least ten times worst that the average photographer,
I did have razor sharp vision when younger).

Instead of giving up on photography I use focusing aids, a 35x loupe
for 45. I count it as a big blessing that I can still rack in an image
across the entire frame with simple tools. It is another blessing to
get a really big image into PS to blow it up and look at the detail.


Thanks for the understanding, kind words and your time.

[AutumnJazz]

Do lenses even resolve enough lp/mm at F16 and smaller to even warrant something like this?

The page linked below seems to indicate that defocus and diffraction severely handicap any lens' resolving power.

[rvhalejr]

Do lenses even resolve enough lp/mm at F16 and smaller to even warrant something like this?

The page linked below seems to indicate that defocus and diffraction severely handicap any lens' resolving power.

I am a subscriber to the theory that the effective diffraction limit is twice the value often cited by what I believe is the standard calculation. I've seen absolutely no problem with that or defocusing when using the loupe/plenum technique, other than vastly improved control over the final image and far better insight into the finer points of LF photography, simply because everything is resolved so much better.

That you have to ask that question is very, very sad. The zealots seemed to have convinced us that at f16, one of the better resolving settings for published lp/mm data are of little value in the field with a view camera.

My experience is that the device at f32 is very useful (often with a
hood of course) and may be so even smaller, perhaps including camera obscura (I'm now putting on body armour and helmut).

The "something" referred to is a product, part of a "business", that does not make enough profit to pay for my 4x5 film (which under a bright light has a circus mirror emulsion reflection that even the critics admit disappears when pulled and taped down for a fluid drum scan).

I can honestly say I love using the 35x loupe/plenum combination and NOT ONE HAS EVER SENT BACK FOR A REFUND which I would be more than happy to do if it does not suit any photographers technique or needs (absolutely no problem).

It may be possible that there is an Ideal replacement for the bright ground
glass and Fresnel lens combo I use, perhaps negating the need for the plenum.

But at this moment I do not believe that is generally the case. Even if it
were, the 35x loupe would still work the same (in theory anyway).

One thing I'd like to see done is the taking and publishing of the measurements for defocusing (I'm assuming you mean the Depth of Focus {DoFocus} at the Emulsion Plane after it has been carefully aligned with the Image plane for the final Image Rack-In).

If you (or an author, instructor, etc.) would like to do that I think it would be great. If nothing else it would better define the DoFocus envelope under various conditions (including, hopefully, a few test cases with basic movements that I use).

I would like to see more information published on the use of small optical pocket microscope systems to evaluate and characterize Optical LF systems.

I've never believed that characterization of SLR, MF, LF and ULF systems would (as the critics would have us believe) be EXACTLY(sic) the same. Yet It seems that the opposite may also be true, that there should be some reasonable similarities while giving recognition to the limitations imposed by optical boundaries.

Sadly, I do know that I'll never be able to investigate all the optical opportunities the Loupe and Plenum combination affords. My hope is that others will and thus provide a generally accepted incremental improvement in the discipline in some small way.

I'm also burdened by the knowledge that the work CS may
have been deprecated here. I ask forgiveness as I forgive those who may be imperfect as I certainly am.

All the Best to You and Yours and God Bless ALL on this first Sunday of the New Year.

[AutumnJazz]

Whoops, forgot my link... http://www.largeformatphotography.info/fstop.html

[rvhalejr]

Thanks !!! A perfect citation !!! I did not want to see this painful comment on the first page though:

... Film flatness is more problematic with LF than with other formats ...

Amen

I'll be looking at the pro/con issues in hopes of aggregating a generally acceptable summarization.

[rvhalejr]

Does any one have a source for really flat 4x5 ?

Out-of-the-box Really flat film test:

1.) Place on a glass table top, gently tap the emulsion surface with
a ball point pin. (Hint: The reflection off the emulsion should not change
where the tip of the pin touches the film).

2.) The reflection off the emulsion under bright light should look like a
typical hand mirror (no circus mirror distortion around the edges).

3.) For the mechanically inclined, a .0015" feeler gauge should not
be able to slip in between the glass table top and film. Flattened
household aluminum foil is ok (it runs between .0005 and .001")

Photographing the mirror like reflection and posting the image gives
validity to test. It's not easy getting a useful digital image of
a near perfect LF emulsion reflection.

I just wanted to share that I used a 2x flip-down magnifiers on my
crappy old D100 and S2 .,. seems to help .,.

[bglick]

I am trying to stay away from this thread, but nothing worse than mis-information still being spread.....


> Photographing the mirror like reflection and posting the image gives validity to test.


The only validity to photographing a films surface is.... you will potentially demonstrate that film does not have a front reflective surface, and therefore will NEVER reflect a perfect image like a front surface mirror will. Film was never designed or tested for wave front errors.


To get to the beef of what you are trying to ascertain.... you should test some film holders for plenum flatness. Record values at different XY coordinates. Use a high tolerance straight edge (<.001" per 6") and dial indicator, .0005" or better. The set-up is called a depth gauge, where you plunge the dial indicator down until it makes contact.

Next, insert film into film holder, run the same test at the same coordinates. Use a good light, you can easily see when the guage pointer makes contact with the film. (of course, you must account for film thickness) These are the results that matter - not using film as a mirror . With good 4x5 holders, the film probably will not bulge more than .006" vs. the previous plenum readings. At f16+ apertures, errors in flatness at this level will be undetectable in the final image, even when imaging flat subjects. See Depth of Focus above...

[Nathan Potter]

Well I'm also trying to stay away from here. But there are other techniques that can be used to check film flatness. They involve interferometry. The simplest would use an optical flat and a monochromatic light source. You'd need an 8 inch diameter flat to cover 4X5 film. You could use a sodium lamp. Since film is very flexible you can't place the flat right on the film surface since it will (the film) deform to the flat. So you need to suspend the flat just above the film say by .020 inch. Then count fringe displacement. I say this but I'm not sure that most film has adequate reflectivity for this technique as bglick has mentioned.

An entirely more elegant method is to use a projection interferometer such as manufactured by Tropel Inc. Again film reflectivity may be an issue.

rjhalejr, examining a reflected image of a straight line say from a bright lamp could only yield qualitative information about the degree of flatness and not real hard numbers. And again your concern is how the film sits in a standard holder so the dial gauge scan of film in holder as suggested by bglick seems like a feasible way to collect some decent data. But even with that technique you don't want to depress the spring loaded back.

Nate Potter, Austin TX.

[bglick]

> Then count fringe displacement. I say this but I'm not sure that most film has adequate reflectivity for this technique as bglick has mentioned.

> An entirely more elegant method is to use a projection interferometer such as manufactured by Tropel Inc. Again film reflectivity may be an issue.


Nate, you correctly point out, all these methods are dependent on film having a single reflective surface. I can assure you, this is not the case. Film is partly transmissive, otherwise rays would not hit the film grain below the surface. So light scatter and light reflectivity can ocurr at many depths. I never tested this scientifically, but its just obvious...

More importantly, the mechanical testing method is so simple, and has accuracies in the magnitude of 10x greater than what is required, and it cost $100....why look any further?