Diaphragm on projection lens

[cyberjunkie]

I got very taken by projection lenses after I successfully adapted an Isco Ultra-Star MC 80mm (gold, blue star/line) to a large M65 helicoid. If I ever find a brass helicoid for cheap I wqill redo the adaptation, but for the moment I am quite happy with the job.... and extraordinary happy with the performance of the lens. Despite being about f/2 (maybe even f/2.1) this Isco cinema projection lens provides for a wonderful mix of good sharpness and beautiful, smooth, buttery bokeh.
I am profoundly impressed. To the point of thinking about giving this lens, or a similar one, a new diaphragm.
I believe that this lens in particular does not have enough space between the front and the back optical "cells", but I have seen golden Isco or Schneider optics that were "customized", giving them a new iris. This means that at least some cinema projection lenses have an optical layout that offers enough space to offer a diaphragm. This should be even easier with other kind of projection lenses. I have at least one aplanat and a few Petzvals that should allow for an easy customization... with the right tools and the needed competence!
Though for the moment I would like to try something simple, like fitting an iris in front (or at the back) of the barrel of a projection lens.
In my case most of my projection lenses have a focal that allows to convert them to M42, which allows to use them either on a Pentax DSLR, my Sony Alpha 7 II, or on a m4/3 camera.
Having the back of the helicoid in M42 thread, whatever the size of the helicoid itself (be it M65, M58 or M42), allows to fit a pre-made iris with an M42x1 thread at both sides.
I am not sure if this is the best solution, though.
Ideally the iris should sit where the optical rays cross. I don't remember the name, it's called something like the back nodal point, if I remember correctly.
A simple optical system, like an achromatic doublet, can have the iris both in front or at the back of the lens, depending on the orientation of the convex side. The Imagon is convex at the back, and has the diaphragm at the front. The Pentax Soft Focus 2.2/85mm is convex at the front, therefore has the iris at the back of the optics, protected from the environment. Very likely the second option is not the best, but it's more practical and allows to contain the length of the barrel of the lens. In both cases the diaphragm is kept very close to the glass. Putting the iris at the back of the helicoid doesn't sound right to me...
In fact, the Isco or Schneider cinema projection lenses (or their anamorphic attachments) at times are sold with a front "stop" that reduces the size of the entrance pupil, with the practical result of making the lens slower.
Unfortunately this kind of solution is both more expensive and more complicated. Large iris blocks on AliExpress cost much much more than the dirty cheap M42 irises, and adapting them to the front of the barrel needs some mechanical work, which would very likely "damage" the physical state of the lens in question.

Having said all that, which solution did you come up with?
I'd be happy with suggestions, caveats, practical guides, but I would also be grateful for simple pictures of examples of adaptation.

Ciao

Paolo

[calvin83]

The iris is ideally placed at the optical center of the lens. For some of the lenses, the optical center can be located inside one of the element.

Can you guess the optical center of the following lenses? The one on the right is pretty easy to guess as it is close to the typical double Gauss we see everyday.

[cyberjunkie]

Hahaha... not as easy at it seems
The last design is, as you say, a typical double gauss, so the iris should go in the middle between the two "cells".
For the two first two layouts I would reduce the complex design to a basic triplet. Not so easy again, cause TTH used to place the diaphragm either behind (more common) the central element, or in front of it (less common).
My best guess for the first would be to place the iris in the middle between the two negative (concave on both sides) elements. To be more precise halfway between the fifth and sixth element.
The second is even more confusing. I would go as close as possible to the back of the first negative, double concave element (fourth from the front).
I wouldn't be surprised if the practical results would dictate another configuration...

I am curious to know to which lenses belong the three cut-outs you posted.
BTW, my Isco should have the third optical layout. Not enough space for a diaphragm.
It would be nice to see the evolution in optical design from the late Kiptars (likely still Petzval design) to the most recent projection objectives for cinemas.
I am pretty sure there were some lenses that at some point even had cemented elements, despite the high temperatures involved.

I forgot mostly anything I knew about optics. My superficial studies about this discipline happened too long time ago. Though I remember well enough that the symmetrical (or almost symmetrical) double gauss, like for example all the Plasmats for large format and enlarging, are not too sensitive to the spacing between the two halves of the optical system. I have personally seen the same cells used for both taking lenses and enlarging lenses. One in shutter and the other in barrel. The lens sold for enlarging negatives had no washer between the elements, while the taking lens had a washer at the back. I am absolutely sure. I wouldn't place my hand on the fire regarding the placement of the distancing ring. I am kind of sure it was at the back but I'm only 80% sure it was on the Symmar. The lenses were a Symmar-S and a Componon-S, or maybe an Apo-Symmar and an Apo-Componon, more likely the former than the latter. I am pretty sure it's possible to confirm my findings searching for similar stories on the Web.

[calvin83]

You can find the diagram here http://www.transferconvert.co.uk/cinemania/ESanamorpH.pdf .

[cyberjunkie]

[dan_]

I did quite some diaphragm adaptations for projection lenses, but the only one presented on this site is the Visionar 100mm here:
http://forum.mflenses.com/forcing-the-visionar-100mm-f-1-6-to-accept-a-diaphragm-t68374,highlight,%2Bvisionar+%2B100.html

And some results here:
http://forum.mflenses.com/visionar-100-mm-f-1-6-and-speed-booster-first-shots-t68540,highlight,%2Bvisionar+%2B100.html

[simple.joy]

[cyberjunkie]

Great conversion of the Visionar 100mm, and beautiful pictures!