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...5,6-150mm.html

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



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:



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