Flange focal distance for lenses mounted in Prontor shutters

[mikec]

I was recently searching the web for some lens info and noticed on one page that I can no longer find, that the lens table in question had flange focal distances quoted for Copal and Prontor shutters and that there was a difference between the two. The ffd is pretty much academic for a ground glass user but for a point and shoot with a helical focuser it is important that the infinity mark is accurately located. Does anyone have a document or link showing what this difference is?

My specific need is to know what the difference is for an Apo-Grandagon 35mm. I have a micrometer and have tried to measure the difference by comparing the distances as follows:

The logic I am using is that if the ffd for the two shutters is the same, then the distance between the front of the lens board on which the shutter rests and the rim of the rear lens block should be identical. Does that sound right?

So I measured that distance with the rear block mounted in its Prontor Prof. 01S and also when mounted in a Copal 0 (normally has a Apo-G. 55mm). The problem is that the jaws of my micrometer are not long enough to take the measurement directly, so I had to place a metal ruler across the rear block to extend the plane.
I tried to use the same lens board points and ruler segments to limit variations but the measurement was not easy and so subject to error.

That said, there is a clear difference between the two of approximately 2.5mm, the Copal being the smaller, so I am asking for some better data.

I could move the 35mm to a Copal 0 for which the ffd is documented but I would prefer to have the choice. I can't simply switch the elements to that of the 55mm as that is shimmed.

Thanks for any pointers you can give.

[Pfsor]

The ffd is pretty much academic for a ground glass user but for a point and shoot with a helical focuser it is important that the infinity mark is accurately located.

Thanks for any pointers you can give.

No. You should never rely on some nominal values of the FFD of your lenses when constructing your P&S camera. The value is never as precise as needed for good infinity focusing. Not only is each lens slightly personal in this distance but it is difficult to build the camera box for the exact value you want with all manufacturing tolerances adding up as you go.
The best is to use a construction that allows you to fine tune the FFD on the camera itself. Fotoman was constructing all of its P&S camera that way too. Amateurs often ignore this constructional fact and their constructions suffers from it.

[Pere Casals]

I was recently searching the web for some lens info and noticed on one page that I can no longer find, that the lens table in question had flange focal distances quoted for Copal and Prontor shutters and that there was a difference between the two.

Yes... https://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/symmar-s/data/1,5,6-150mm.html

If mounting, for example, a Symmar-S 150mm in a copal 0 vs compur 0 you have 0.4mm difference.

187528

But as Pfsor says best is allowing some kind adjustment in your design.

[Pfsor]

Pere,
you're good at finding info on the net, I noticed that many times. Admirable!

[Pere Casals]

Pere,
you're good at finding info on the net, I noticed that many times. Admirable!

Pfsor, not that good, I could not find the right parameter in shutter specs tables. I guess that the small mismatch may be calculated from some dimensions in the drawings:

187529

But I guess that if that value is not in the tables it should be because of its practical irrelevance. Perhaps ambient temperature may have a greater effect, I guess that design athermalization may priorize compensation of aberrations rather than conserving exact focal.

[Dan Fromm]

Papi, Pfsor, I compiled my list of useful links so that people wouldn't have to be good at finding things on the 'net. Its there, use it.

A propos of flange-focal distances, I once bought ten (10) AGI F.135 aerial cameras. Each held two (2) 38/4.5 Biogons. The AGI F.135 body is a beautiful die casting and is only lightly machined. The lenses in their shutters (F.135 electric shutters, unusable on all other cameras) are bolted to the body's backbone. Each lens-shutter pair was marked with the lens' measured focal length (38.3 - 38.8 mm) and flange-focal distance. Each lens-shutter pair had a shim, with the shim's thickness in hundredths of a mm marked with a pencil, that gave it the necessary flange-focal distance. Another way of making the FFD slightly adjustable in order to get good inifinity focus with real lenses. There's no substitute for conforming to reality.

[mikec]

Thanks for the info. I agree that for optimal registration each lens should be measured individually.

[Pere Casals]

I compiled my list of useful links so that people wouldn't have to be good at finding things on the 'net.

This is an impressive vault of information...

...but Dan, what parameter in the shutter specs table can be related to that small shift in the FFD for the same lens ? ...or it can only be be known from drawings ?

[ic-racer]

I suspect the small differences in FF distance have to do with differences in the way the various sized threads on the front and rear cell housings are machine to fit the different shutters.

[Emmanuel BIGLER]

...
My specific need is to know what the difference is for an Apo-Grandagon 35mm. ... Thanks for any pointers you can give.

Hi !

Please find attached as an image a scan of the official Rodenstcok documentation with flange-focal distances (FFD) for the 3 apo-grandagon lenses.
As scanned & published by Paul Butzi with permission from Bob Salomon.

For the apo-grandagon 35 mm
FFD = 43.2 mm for #0 shutter
FFD = 41.1 mm for prontor 01S

[Dan Fromm]

I suspect the small differences in FF distance have to do with differences in the way the various sized threads on the front and rear cell housings are machine to fit the different shutters.

ic, I think that the differences are in front and rear tube lengths. The sum of the two lengths has to give the necessary distance between the cells, but one cell can be set a little more, the other a little less, deep in the shutter.

Papi, most, perhaps all, lens specification tables give flange-focal distance. Flange-focal distance is meaningless for empty shutters.

[Pfsor]

Thanks for the info. I agree that for optimal registration each lens should be measured individually.

No measuring can replace an adaptive construction that can fine tune the lens to the correct FFD. Without it the more precisely you measure, the more precisely you need to build the rest of the camera body, and you will surely not grind it to 0.001mm tolerances. Shims are the most barbaric solution to the problem still demanding a lot of measuring. A continuously tunable device is much better. Take your inspiration from Fotoman cameras.

[Pere Casals]

Flange-focal distance is meaningless for empty shutters.

Yes... but there is a (it looks not published) dimension, (say) from rear cell stop top flange (limiting the screew in) versus the shutter-board flange. It should be this dimension the responsible for the FFD missmatch in different shutters, as the intercell distance is well specified...

It looks that a shutter brand may mount the cells (some 1mm) a bit forward or back compared to others.

What I find a bit surprising is that this was not completely standard, as threads and inter-cell spacing are... perhaps only what was important was fully standarized (threads and inter-cell).

[Dan Fromm]

Yes... but there is a (it looks not published) dimension, (say) from rear cell stop top flange (limiting the screew in) versus the shutter-board flange. It should be this dimension the responsible for the FFD missmatch in different shutters, as the intercell distance is well specified...

It looks that a shutter brand may mount the cells (some 1mm) a bit forward or back compared to others.

What I find a bit surprising is that this was not completely standard, as threads and inter-cell spacing are... perhaps only what was important was fully standarized (threads and inter-cell).

Yes. What matters is threading (diameter, pitch, form) and tube length.

Randy, I downloaded El Hassan's dissertation years ago. There's much to be said for book learnin' when shopping. Much safer than just spending and praying.

Pfsor, the F.135's backbone is a large flat plate. The camera body is a die cast box with the backbone in the middle. Many of the subassemblies attach to the backbone, the others attach to the box. Sorry if I haven't explained it well enough. In that camera, shims do perfectly well and are immune to vibration. Remember, aircraft vibrate. Incorporating a focusing mechanism is a much worse solution in that context than using shims.

[Pfsor]

Dan, I'm not talking to the OP about aircraft cameras. The guy is going to construct his P&S monster. I that context shims are much more difficult solution than the one I spoke about. Beside the fact that vibrations cannot undo Fotoman cameras infinity focusing either. If you want him to take inspiration for his PS camera from F135 air camera you're free to reveal to him all you want.

[Jac@stafford.net]

A challenge for me in making my first wide-angle 4x5 (http://www.digoliardi.net/super-wide-4x5-1.jpg)was getting the depth right for the helical focusing mount at infinity. Finally I found so-called Silicone Sponge Tape which is closed-cell, modestly compressible (~3mm). I put it between the film holder and camera body, and turned the back's fasteners until I got the right depth which was checked using aerial focusing through a hole in the ground glass. About every two years I replace the tape, refocus.

[Pfsor]

A challenge for me in making my first wide-angle 4x5 (http://www.digoliardi.net/super-wide-4x5-1.jpg)was getting the depth right for the helical focusing mount at infinity. Finally I found so-called Silicone Sponge Tape which is closed-cell, modestly compressible (~3mm). I put it between the film holder and camera body, and turned the back's fasteners until I got the right depth which was checked using aerial focusing through a hole in the ground glass. About every two years I replace the tape, refocus.

That's one way how to achieve the correct FFD at infinity. The advantage of continuous adapting to the FFD of the lens is principal. But why don't you just replace the sponge tape with small strong springs? Once the position is found and secured you're free from replacing the tape and still you can re-adjust the FFD at will.

[Jac@stafford.net]

That's one way how to achieve the correct FFD at infinity. The advantage of continuous adapting to the FFD of the lens is principal. But why don't you just replace the sponge tape with small strong springs? Once the position is found and secured you're free from replacing the tape and still you can re-adjust the FFD at will.

Good idea. The sponge-like tape also acted as a light seal. Springs would be cool but I am not smart enough to know how to incorporate them into the back and also have a perfect light sealing, and I was, for better or worse, committed to simplicity and a shallow body.

[Pfsor]

Good idea. The sponge-like tape also acted as a light seal. Springs would be cool but I am not smart enough to know how to incorporate them into the back and also have a perfect light sealing, and I was, for better or worse, committed to simplicity and a shallow body.

I put the springs on 4 rods and cover the whole part with bellows made like short bag bellows. But later I used a different method without springs and still adjustable, a "proprietary invention". :) Cheers!

[Jac@stafford.net]

I put the springs on 4 rods and cover the whole part with bellows made like short bag bellows. But later I used a different method without springs and still adjustable, a "proprietary invention". :) Cheers!

Good information. Thank you.

I had mere millimeters to accommodate. I think I made the right design decision. Remember that my camera had a focus helix, and when in doubt, a ground-glass.

(the cherry-wood body was made to mm accuracy by my friend, Don, on his micro-precision planer with my suggestion for a mm adjustment of my choice which was to be 2mm short of ideal, the rest to be made up with a back gasket.)

"When the answer is found it will be simple" -- my uncle, Domina Jalbert (https://en.wikipedia.org/wiki/Domina_Jalbert)

[Pfsor]

Sure, having the gg can be used to our advantage too. Putting the focus helix at a shorter than infinity FFD and then just focusing correctly the infinity at the gg, even marking the newly found infinity at the focus helix is also a possibility. Then you don't even need to check the gg, the marking is sufficiently precise (as even eventual 1 or 2° over turning of the helix translates to a very minimal FFD change, precise enough.)

[Pfsor]

(the cherry-wood body was made to mm accuracy by my friend, Don, on his micro-precision planer with my suggestion for a mm adjustment of my choice which was to be 2mm short of ideal, the rest to be made up with a back gasket.)



While thinking of it - what you said, should be written in golden letters! As a warning to all those who want to build a camera thinking that it is enough to know the FFD and then build a box of the right thickness for it.
In reality, it is very difficult to measure all the parts that make that box and to make it precisely for the given FFD without falling in some practical difficulties on the way (see the OP). Even more so, when installing a 35mm lens as the OP wants. That lens will need very precise plan parallelism difficult to achieve at once with all the parts involved.
An adjustable construction is necessary in amateur manufacturing.

[Jac@stafford.net]

I am thinking of Dan From's many posts which concern his experience with military RECON cameras and lenses. A very good friend of mine was an air recon tech who dealt with Voodoo and U2 films. He was always impressed with the camera and lens technology and he added, "Nothing can replace the photo interpreters..." The human's capacity then (the Sixties) could recognize and accommodate lenses ever-so-subtle errors. But that was before digital. :)