Psuedo helical focussing - possible?

[bglick]

Struan

> Since then, I've had the luck to work with several similar mentors who had the happy knack of making what I wanted rather than what I asked for. At present you seem to be thinking out loud in a vast sea of options, which is fine but they can't be distilled down to a final design until you start narrowing your options.

I can certainly understand how you feel that way from this massive wandering thread, but I have narrowed it down, to exactly what I want. Admittedly, this was occuring through the thread, and while speaking to camera builders, lens makers, etc.

> The devices I pointed to are designs suitable for precisely moving a fixed lensboard with both lenses mounted on it. If you are going to the trouble and expense of getting matched lenses this is the route to take:

I did look at the links.... and although I certainly don't disagree with your position, it comes to down to practicality of finding someone to assemble these pieces, the weight and size, the method to utilize these fine focussers.... how much loss would be in the telescoping lens board? Where is the physical space to do all this in a tight compact field camera? Get my drift here? If this was a studio camera whereas were designing them to be sold in volume, then I am sure an better system can be had....

> if you put them in seperate helical focussers there's no real need to match them.

I learned this the hard way.... lenses vary tremendously in fl's , even in the same batch. If they were not paired, then I would be allowing a potential issue creep in, which I have no way of testing, hence why I have the factory laser test the lenses and pair them. Once again, try to eliminate all the controlable obstacles.

> Wide bore helical tubes that would do the job are available, or can be made, but something that encompasses two 47 mm LF/MF lenses and their shutters is going to be bulky, heavy and expensive.

Yep, I have to stick to Copal 0 shutters, and sync them externally.... not easy, but doable, I have had success at this in the past.

> A final simpler option would be an adjustable scissor strut like those found on countless focussing folder cameras. Grimes or any other instrument-quality machinist can make a version of any of these for you if that's what you really want and need.

Yep, this was my first concept.... till I realized how difficult it is to keep scissors straight on 6" wide lens board. With single lens, with optical axis in middle of board, this is perfect, but with two lenses, both off axis from board center, all slight mis alignments are exaggerated. With the factory mounted helicals, I can always see them on the gg and control them seperatly with a very acceptable geared ratio. I still can't comprehend a better system for this application.

> On the other hand, if you can live with f11 or smaller, and only focussing down to 20 feet, there is no need for a focussing device at all. Assuming the images will be 6x8, the conventional hyperfocal distance for a 47 mm lens at f11 is about 4 m, or just over 13 feet. Even at f5.6 you're only at 26 feet. On the handheld you could just live with zone focus

I need the ability to focus at certain distances, albeit a tight range, say 9 ft to 18 ft..... but quite often I need the sharp focus directly on the subject, such as in portrait type pix. At f11, and no higher, resolution at the focal point is extremely sharp vs. the near and far.... hence why I worked so hard for focus in this camera. Its also why I called this rig a bit of a hybrid.

> (I would set the registration to the hyperfocal distance for f5.6, then stopping down just makes things better). On the full-blown camera you could use a geared rear focus to fine tune things or to focus closer. If you must have really fine focus, you could gear down the rack-and-pinion drive on the back, or incorporate a small linar drive from an optics catalogue.

Rack and pinion drive on view cameras is no where near accurate enough for focus this fine. I tested it on a few precision view cameras. Again... even if I found a precision sinar with better fine focus, I still can't imagine it outperforming the helical mounts from the lens maker? I guess I am confused why you are bucking against the helicals... I really beleive the other methods would work also, but trying to find someone to do all this work, testing it, reliability for years to come, the size and weight of the camera, etc. ..... in the end, it seems the helicals keep winning, they are time tested for this exact task.... if you machine the lens board mount so its square to film plane and at proper distance, everything should consistently work.

Bill

[Dan Fromm]

Bill, take a look at how microscope focusing mechanisms work. They give finer control than you need, and millions -- literally -- have been made.

[bglick]

Dan... I agree microscope focus mechanism are very high precision, and more than I need. And I could easily hack one up for its gear system.... but as I mentioned to Struans post.... now, find away to put this on a lens board assembly whereas there is only 47mm of space to work, two lenses, etc. Now the geared mechanism becomes the easy part..... now the customization is..... what exactly is this geared system moving? So you gain one off-the-shelf item only forcing you to custom make another part to utilize its function. This once again is the beauty of the helical, its centered around the lens and removes that huge variable....., which makers of folder cameras never had to contend with, as they had the lens axis in the middle of the board they were controlling....

[Struan Gray]

Bill, I'm avoiding individual helicals because you said you wanted to avoid then if you could. They'll increase the seperation of your lenses and the amount of fiddling you have to do before you take a photo. I and others are just pointing out that if you want to explore other options, they are out there. As Dan (and I) said, there are better rack and pinion focussing tracks available for $100 or so if you need them, or you can use a micrometer or a differential screw drive like these, pushing on a linear bearing if you want micron accuracy.

To me, it makes sense to shim the lenses once and use a single translation mechanism to move them together as a unit. I agree that a helical drive is simple, intuitive and robust. If you have the budget, go for a single wide-radius helical with a bore large enough for both lenses. You'll get guaranteed synchronised focussing, the gearing will be even better than on a helical for a single lens, and you halve the time spent looking at ground glasses. Finally, you can use the same focusser for 47 mm and 80 mm lenses, reducing overall system compexity. I'm sure Grimes could make such a thing, and if not optics houses like Melles Griot or Edmund Optics certainly can.

Finally, if you do go with a single focusser, consider a single shutter like the Sinar shutter or a big 'ole alphax. That would let you use lenses in barrel (no problems getting them close together) and ensure synchronisation of picture taking and flash. The only downside would be a reduced max shutter speed.

[Struan Gray]

I just can't help myself.

It ocurred to me that you might not understand what I mean by a 'large radius helical'.

Imagine a 150 mm square lensboard with a 140 mm diameter hole in it, surrounded by a cylindrical lip that is threaded with a male 140mm x 1mm pitch thread.

Now screw onto that a circular collar with a matching female 140 mm x 1 mm thread on the rear, and a female 140 mm x 1 mm left hand thread on the front.

Now screw onto that a custom board with a 140 mm x 1 mm left hand male threaded lip on the rear and a regular lensboard attachment on the front.

In one or more corners, mount a pin that is fixed to the rear board and passes through a tight hole on the front - this stops the front and rear boards from rotating with respect to each other.

Mount your lenses side-by-side on a regular lensboard, and attach them to the front of the contraption.

Rotating the collar moves the boards 2 mm apart for each turn. Add a focus handle to the collar, and use only one corner pin so you get one whole revolutoin, and you have a device with enough motion to focus the 47 mm lenses to 3 feet, and the 80 mm lensee to 10 feet.

You'll need a spacer to increase the infinity distance for the 80 mm lenses (make it part of their lensboard). You may need to include a concentric light baffle inside the tube.

[Struan Gray]

OK, OK, make the hole 138 mm. Or 136 for safety's sake.

[bglick]

Struan, thanks for your presistence in helping.....

> Bill, I'm avoiding individual helicals because you said you wanted to avoid then if you could. They'll increase the seperation of your lenses and the amount of fiddling you have to do before you take a photo.

Ahh.... OK, I see.... well, in the end, the compromise in center lens spacing was small after hacking up the factory helicals a bit......but something I still want to consider as center lens spacing is critical....

> I and others are just pointing out that if you want to explore other options, they are out there. As Dan (and I) said, there are better rack and pinion focussing tracks available for $100 or so if you need them, or you can use a micrometer or a differential screw drive like these, pushing on a linear bearing if you want micron accuracy.

I fully comprehend this....

> To me, it makes sense to shim the lenses once and use a single translation mechanism to move them together as a unit.

And I agree with this too, always have.....more below....

> It ocurred to me that you might not understand what I mean by a 'large radius helical'.

Yep, Struan, I did miss your point... I just plain did not visualize a huge helical going around the two lenses.....sheeeesh, that is just plain brilliant! You still amaze me.... I will discuss this with others to evaluate pros and cons.. and I may devise such a rig, as single focus mechanism is surely desireable...... but, there is one issue that may prevent me from going forward with it. With the dual camera system I am making....i.e. meaning the lens boards will switch between handheld version and veiw camera version.... I would have to maintain the same lens board for both...... it's possible, this needs to be explored...and also the ability to use rise and tilt on the view camera arrangement may cause some issues here with massive helical, unless its actualy part of the lens board....

> Finally, if you do go with a single focusser, consider a single shutter like the Sinar shutter or a big 'ole alphax. That would let you use lenses in barrel (no problems getting them close together) and ensure synchronisation of picture taking and flash. The only downside would be a reduced max shutter speed.

Ahhh, once again, you nudged me in the right direction..... Although no modern focal plane shutter will work for an image this big, sinar does make a 6x4.5 focal plane shutter. Now, I can affix a pair of these to a rear lens board, and it will mount on and off with each lensboard, so they can be used for other lenses. Not sure I can get the electronics wired externaly, or just how much electronics is required from their digital cameras.... but if this works, it may be fabulous, as I now have shutters up to 1/2000 th, perfect for outdoor shooting of fast moving subjects! It is much easier to sync electronic driven shutters vs. full mechanical shutters. The old shutters, even 4x5 are not big enough, although one would fit nicely behind the lenses...

Which made me think.....any leads on a large focal plane shutter that can cover about 120mm, 5"? The old graflock versions of these are just way too clunky, although some were fast, 1/1000 th on 4x5 format, which is good enough. Huge benefit to one single shutter, perfect sync is guarantted, but I must be careful not to add vibration which will degrade image quality quickly.... I can't seem to find any at the link you provided....any other sources? Does any place custom make these?

With any luck, I can now have one single large helical foccuser (vs. 4 hellical mounts) so all lenses can be purchased in barrel, then a single fl shutter behind the lens. I must admit, if this pans out, it will be a better system than my previous system.....

THANKS STRUAN! Keep it coming! :-)
Bill

[bglick]

Ahhh... LCD shutters.... it seems they would be perfect.... even if they cost me 1.5 stops of light, but perfect sync.... I may have to start a new topic to see if there is any experience users of such.

[Kevin Kemner]

Wow Bill,

This thread has been all over the place. I reread through the threads above and then took some time to do a little research and want to throw one last set of thoughts out for you. First off, I work with engineers and industrial designers all the time and will try to approach your problem as they would.

So lets get down to basics. You are trying to build a 6x17 medium format stereo camera based using either 47mm or 65mm focal lengths. Your primary need for this project is an as yet unstated project with focal distances between 15 and 20 feet. You would like to have a second use of the lens assembly for a hand held hyperfocal landscape stereo camera. You have expressed concerns about film alignment precision and the ability for very small focus movements. You intend to use ground glass focusing.

Okay, lets look at limiting factors. We do not know how you intend to reproduce your images. As you are looking at stereo photography we could assume that you will be using a viewer of some kind to look at these images. The images will probably be small in that case. As we don't really know how you intend to reproduce these images lets wait for this information.

We do know that you intend ground glass focusing. At the format size you are using, ground glass focusing is incapable of matching the resolving power of the lenses. This is a weak point in your camera if you are obsessed with precision. One reason that MF view cameras are rare is the inherent inability of ground glass focusing to match the rangefinding or split screen focus systems of current medium format cameras.

Ground Glass and film plane calibration. Its pretty common that ground glass backs and film holders are not in the same focal plane. This is an easy fix and all you need is some precision brass shim, a 12" bar of brass flat stock and a plunge micrometer. The fix can be done in less than a day and will show significant improvement in your images. As for consistant lens axis to film plane alignment, if you intend to use movements the best you can hope for is close alignment rather than perfect alignment. It is practically impossible to have a view camera with perfectly aligned axis.

Okay, what works in your favor? Surprisingly, DOF and Hyperfocal distances are all in your favor. I ran some dof calcs for your camera with the lenses at f5.6 and as near as I can tell your camera should have a dof range of at least 10' through 25' for your project. It gets better at f11. In fact the dof for this camera would be so good that you good be focused a good 2' off of your subject and it still would be in sharp focus at f5.6. So, where does that leave you?

Much better off than you think. For your close focus project all you need is a monorail camera capable of mounting your film back, of using a bag bellows, and with a large enough lensboard to mount your two lenses. Its not glam but it could be cheap.

Anyway, give up on the complicated solutions and go simple and straightfoward. Find a camera that meets your needs, get a couple lensboards drilled out and get to work.

Now if you want a field camera check out these two sites.

http://phsne.org/stereocameras/STEREOFORMATS.HTM

http://www.ghouse.com/daniel/stereoscopy/equipment/

Sorry they're not links.

Kevin

[bglick]

Kevin.....

> This thread has been all over the place.

yep, this is what happens when great minds collaborate! More than one way to skin a cat!

> So lets get down to basics. You are trying to build a 6x17 medium format stereo camera based using either 47mm or 65mm focal lengths. Your primary need for this project is an as yet unstated project with focal distances between 15 and 20 feet.

Focus distances can range based on the fl lenses being used. The two fl's you mentioned are my basic two, next would be an 80mm fl.....so maybe 3 total...but this does not change the focus issue....

> You would like to have a second use of the lens assembly for a hand held hyperfocal landscape stereo camera.

Well, hyperfocal was suggested by many, but not by me.... more importantly, with out making things even more involved in this thread... I have all the DOF issues, cc, magnfication of viewer, losses in reproduction, etc. worked out and proven. In the end, I need these lenses so focus at distances between 9 - 18 ft, as a basic range, maybe up to 27 ft with the 80mm lenses, but not that signficant.... as it's hard to change focus from 12 to 15 ft with any of my proposed lenses without these radical procedures.

> You have expressed concerns about film alignment precision and the ability for very small focus movements. You intend to use ground glass focusing.

The handheld version will be..... set lenses at desired focus distance, look through top mounted viewfinder and fire away, I must trust the focus scales, through much prior testing. As one of the important issues is focussing on a subject, like people, at the distance they are at....so if I have someone at 15 ft, I need to be focussed close to 15 ft, not 12 ft and not 18 ft.... and since the change in this focus range, +/- 3ft represents less than .1mm of lens movement to and fro the film plane...... that was the basis of all this theorizing on the best way to accomplish such precision movements on a non conventional camera.

> Okay, lets look at limiting factors. We do not know how you intend to reproduce your images. As you are looking at stereo photography we could assume that you will be using a viewer of some kind to look at these images. The images will probably be small in that case. As we don't really know how you intend to reproduce these images lets wait for this information.

yep, images are put in mounts, viewed in a viewer with 70mm fl achromat doublet lenses.... but I did not want to get into this, as I was focussing on the subject of camera focus :-) Images will be reproduced also, but it's irrelevant to making a camera focus on intended target accurately.

> We do know that you intend ground glass focusing. At the format size you are using, ground glass focusing is incapable of matching the resolving power of the lenses.

Very true.... but that does not prevent one from focussing accurately on a target.

> This is a weak point in your camera if you are obsessed with precision.

In theory this is a weakpoint in EVERY camera system. No viewfinder can resolve what film / lens can resolve, however, with a 3x loupe on the gg and very tight helical like Struan suggested will allow me to hone in right on the subject...(assuming I have the gg / film plane licked) so maybe if the subject is at 15 ft, I may get it to 14.5 to 15.5 ft.... which is much better than 12 - 18 ft..... and it takes all this focus precision to make this improvment.... I am not being crazy here.... but just getting close to the focus distance you want takes tremendous precision of lens to film position...then as you state, there is still slop in the system.....

> One reason that MF view cameras are rare is the inherent inability of ground glass focusing to match the rangefinding or split screen focus systems of current medium format cameras.

True, and what seperates view cameras from these MF cameras is precision focus at close distances.... not to mention slop in the standards, etc. So much of this will be eliminated in my view camera set up, and definetly in my hand-held arrangement as everything will be fixed, just like an Alpa. But not as pretty of course. Anyway, thats why this camera is more of hybrid, leaning more towards MF vs. LF.

> Ground Glass and film plane calibration. Its pretty common that ground glass backs and film holders are not in the same focal plane. This is an easy fix and all you need is some precision brass shim, a 12" bar of brass flat stock and a plunge micrometer. The fix can be done in less than a day and will show significant improvement in your images.

Kieth Canham tested his 6x17 back in his 5x7 back for quite some time..... he claims everything is within .0001" and set up for 120 film thickness and characterstics. So this was a big break for me, the fact the roll film back maker and the 5x7 back maker perfected this alignment, hence I do not have to re invent this portion of the camera.

> As for consistant lens axis to film plane alignment, if you intend to use movements the best you can hope for is close alignment rather than perfect alignment. It is practically impossible to have a view camera with perfectly aligned axis.

Agreed..... this is the price I will pay for movements, when I use them.... which won't be often, but I like to leave the option open. Hence why I want the fixed hand-held system also, a box with a helical.... no standards.....

> Okay, what works in your favor? Surprisingly, DOF and Hyperfocal distances are all in your favor. I ran some dof calcs for your camera with the lenses at f5.6 and as near as I can tell your camera should have a dof range of at least 10' through 25' for your project.

yep, however, I quite often need infinity in the scene.... just for your information...the way I do stereo calculations is to start with the min acceptable lpmm or cc which is acceptable when viewing the slides (obviously this is the near and far subjects) then I reverse engineer from there, repro losses, then viewer magnification, then f stop, etc..... now all this must fall within acceptable stereo DOF limits, so the images can be enjoyed in the viewer without forcing someone to do extreme convergence (cross eyed) which would ruin the entire experience.... anyway, this sets the entire near / far allowable conditions for the lens seperation and fl I am shooting with. Then I must make the lens focus at a distance to which will allow acceptable lpmm at my near and fars to meet min. criteria, with f stop being the only other variable I have to play with.....however, in this case, the digitars are optimized at f11, which is my limiting factor, (as 5.6 is to limiting in DOF and f16 causes too much diffraction at the point of exact focus) so focus is the ONLY real variable I have left to meet my crietera... and just trying to focus at 15 ft and missing by 3 ft, (.05mm on focus rail), and due to all the other eroding factors, its quite easy to fail at what seems so simple .....and missing focus point will completely throw off my resolution values I am trying to acheive.

However, the other more critical reason for sharp focus is when I shoot subjects like models, whereas I want there eyes in the sharpest focus, so now i am trying to hold +/- 3 inches. Surely this will be a challenge, even in the box camera version (hand held) with no standards, hence all the trouble I am going through to make this unique camera.

> It gets better at f11. In fact the dof for this camera would be so good that you good be focused a good 2' off of your subject and it still would be in sharp focus at f5.6.

You are not using the same lpmm minimums I am using..... 5.6 is not possible...... I can fill you in on the details of this, but it will make this post wander forever more.... I have been shooting MF stereo for 6 years and have seen all this first hand and know my exact goals....but I do appreciate you thinking this through like you have. I know what my two obstacles are to making this camera successful..... 1) abiltiy to focus at intended distance and 2) near perfect lens sync!

> Much better off than you think. For your close focus project all you need is a monorail camera capable of mounting your film back, of using a bag bellows, and with a large enough lensboard to mount your two lenses. Its not glam but it could be cheap.

Even in your example of missing focus by 2ft, it's hard to make a view camera focus to be within this range of your intended target as there is NOT enough precision to move the standard in .05mm increments (which matches up to your mis focus values) ...it's a crap shoot.... I shoot LF also, 4x5 and 8x10, and these are issues I never bothered with....

> Anyway, give up on the complicated solutions and go simple and straightfoward. Find a camera that meets your needs, get a couple lensboards drilled out and get to work.

If it was this simple, I would have done this long ago.... its not that simple for the precision I am trying to achieve, i.e. the on film resolution at the near and far, as well as the ability to focus within inches of my intended target. If you re read my post above about the precision on the fine focus knob of my Toyo VX125, you will understand why it will never happen.... and the VX is geared quite precise vs. most view cameras...maybe a studio camera would be geared a bit better, but this camera is for field use and needs to be handheld, so those cameras are not an option.

> Now if you want a field camera check out these two sites.

I have seen most of these antique cameras... in their day, they were wonderful.....but to take advantage of todays super high resolving films and very high resolving digital lenses.... precision is the order of the day...and if all i wanted was mediocre results, trust me, I would by a 1970's vintage Russion Sputnick MF stereo camera, they made 35,000 of them..... and for their nominal value, $500, and with some tweaking, they can do quite a good job within certain shooting limitations.... but just like anything else, when you try to squeeze the last bit juice from something, the complications increase exponentially, not linearly, like we all hope for.....

A this point, I think Struan "monster helical" approach is most ideal for the focus issue...I will work to make this a reality.... if not, I feel like the factory mounted helicals is the next best option. If I surrender the ability to allow movements, then the Monsta Helical on a fixed box is best...it still allows for gg composition if I desire...I just loose rise and tilt capability, hence why I was trying to make the lens boards work on both....

BTW, the way I know the distance of a subject is with a laser rangefinder... i.e when not using gg focussing..... hence why focus scales will be needed.

At this point, I am trying to wrap up the lens sync issue, and the LCD shutters have me very excited.... as prior to this, I was ready to settle for mechanical sync, which is not easy at 1/125 +. I started another post on this. The application of these shutters may also dictate the final design of the camera....i.e. box version only, or box + view camera.... Its my bet, if these LCD shutters work out, they will be located behind the lenses...and this may cause problems with tilt as a 47 mm lens is already slammed against the gg.....

At this point, if could build in a little rise into the monster helical, I would forgo the tilt and settle on the box version.

Bill