Indicated apertures & element transplants

[BennehBoy]

I have a 300/5.6 lens mounted in a Prontor Pro 3 shutter. I have an opportunity to obtain just the elements of a 165/8 SA lens which I believe requires a #3 shutter.

If I place the 165mm lens elements in the prontor, how am I to gauge the correct aperture?

Correct me if I'm wrong (I expect that I am), but the maximum iris on the shutter should be roughly 300/5.6 = approx 53.5mm (marked 5.6 on the shutter). The 165mm lens requires an iris of approx 20.6mm for f/8 - none of the presets on the shutter are particularly near this. The indicated aperture on the shutter with the closest diameter, at 18mm, is f/16 but by using this as my starting reference I am effectively hobbling myself to not being able to stop the lens down adequately.

Am I missing some fundamental concept here?

I'd dearly like to stay with prontor pro shutters, but if I need to obtain a copal so be it. (I also have a prontor 0S and 01S available). I'll have to obtain another shutter at some point since I have no wish to have to swap elements every time I wish to shoot a different focal length - but it would be nice to test the elements when they arrive, and to know what shutter I need to but/how to adjust one.

[Bob Salomon]

One easy way, if you can find it, Marflex got all the ones we had I think, is to use the push-on aperture controllers for the PP. These are calibrated for the lens and apertures and can be read and set from in front or behind the camera. They were one of the many benefits to the PP shutter system.

However you would be hard pressed to find the controllers in the first place and the correct one for the 165 in the second.

If you can find one that fits the PP 3 shutter then you could remove the scales on it and add your own calibrated scale. For use with the 300 you would then just take the controller off.

Otherwise you have to have new aperture scales made for the shutter itself and the new scale for the 165 would be wrong for the 300.

Why not look for a second shutter?

[BennehBoy]

I have the controller - I guess the real issue is why a #3 shutter, it kind of limits the range of apertures that can be set for an f/8 wide lens.

I guess the question is this, are all PP #3 shutters identical in terms of the iris sizes, but just a different scale is fitted? Same question for copals. The maths would seem to indicate otherwise.

[Peter K]

I have the controller - I guess the real issue is why a #3 shutter, it kind of limits the range of apertures that can be set for an f/8 wide lens.
Older SA 165mm where mounted in a special shutter "MXCRI". Later the lens was mounted in shutter size #3. Also a shutter size #1 with it's small opening should be the right size. But the front- and rear elements are so heavy the threads of a smaller shutter can't bear this lens.

[aduncanson]

This is a recurring question which is particularly frustrating because those who ask almost never are prepared to accept for the answer. The aperture diameter which is equal to the Focal Length/F-Number is not the physical diameter of the aperture mechanism, but the exit pupil diameter which is the image of the physical aperture as viewed, magnified or demagnified, through the front element of the lens from an infinite distance. Emmanuel BIGLER gave a very thorough explanation and a practical, but not simple, approach to calibrating an aperture correctly in the following thread.

http://www.largeformatphotography.info/forum/showthread.php?t=32332&highlight=f-stop

[BennehBoy]

@aduncanson, thanks for the link, I knew my theory would be off the mark somewhere.