How to determine aperture plane

[withnail3]

Hi,
I've just searched the forums but can't find what I'm looking for, possibly I'm using the wrong keywords.
I'm repairing an 8x10 field camera which I got free. It's all fixed up but there was no lens with the camera.
I have a few barrel lenses which I know will cover 8x10 but they have no shutters.
So my plan is to make my own aperture/waterhouse stops which will double up as a shutter.
Only thing I'm stuck on is how to determine the ideal distance from the rear of the lens to the waterhouse stop.
Is it simply trial and error or is there a layman's way of doing this? Maybe shining a light through the lens onto a flat surface with the waterhouse stops inbetween, or something?!
Any help much appreciated

 

[Dan Fromm]

Which lenses do you have in barrel?

 

[withnail3]

Hi Dan,
I have a Rodenstock Apo-Ronar 480mm f9 (which obviously has it's own diaphragm), Industar 37 300 f4,5 (diaphragm has been removed, slight vignetting in the corners at infinity), a couple of obscure projector lenses and some meniscus glass from old telephoto lenses which all cover a bit more than 8x10 at infinity.
I also have a complete set of 4 lenses from an Agfa Repromaster copy camera. I haven't tested these yet but I'm assuming they have a large image circle considering the machine they belonged to.

 

[Nodda Duma]

Look through the front or back of the lens and you see that the light path is circular (that is, it is not infinitely wide). Whatever mechanical edge forms that limiting diameter is the defining aperture. Put your waterhouse stops there.

But it sounds like you don’t have a fully assembled lens. So another approach: If the layout of the lens is roughly symmetrical front to back (that is, the front of the assembly kind of mirrors the back of the assembly), put your waterhouse stops in the middle. Adjust the center airgaps — distance from front and rear assemblies to the stop — for optimal image quality on and off axis. You will need to make a dummy barrel or telescoping barrel to let you accurately move the assemblies back and forth without shifting or tilting. Pain in the butt to do at home unless you have a lathe, but precision optics require precision adjustments.

 

[withnail3]

Thanks Nodda, nice tip and simpler than I thought.
Although I could only use that approach with the meniscus and projector lenses I think, because it would mean cutting the other lenses in half which I know people do but I didn't want to go that far just yet.
I've heard it's possible to place the waterhouse stops in front of or behind the barrel lens in a kind of tunnel construction, but finding the ideal distance requires some calculating..

 

[Nodda Duma]

My way: know what design you have and find the layout online.

The Industar is a Tessar design. The waterhouse will go between the 2nd and 3rd group, roughly centered.

http://lookinlens.com/optical-schemes-inexpensive-analogs-of-the-lenses-carl-zeiss-and-leica.html


Addendum: Putting the aperture out in front of or behind a barrel lens won’t give you the best quality. That only really works for a landscape lens (meniscus and aperture with concave surface facing aperture). You find the ideal distance by examining the in-focus image at the corners and adjusting the distance until the sagittal and tangential aberrations (parallel and perpendicular “smear”) is balanced.

 

[withnail3]

Ok thanks. I may go that way after some experimenting with the meniscus types.
Think I'd rather cut the Industar than the Ronar ;-)

 

[Nodda Duma]

Good luck!

 

[withnail3]

Maybe I should have explained further....my waterhouse stops are a hole cut into a plastic disc. The disc is fixed to a shaft which I spin by hand so it acts like a shutter too. It's about 1/10th of a second according to a test with my dslr.
So I have interchangeable discs for different apertures. The idea is to combine aperture and shutter into one thing allowing me to play around with different glass by adjusting the distance between the disc and the rear (or front) of the lens.
Anyway, I'm probably over-thinking this bit and just need to try it at different distances.
With the Ronar I can use the diaphragm already in the lens and just use my disc as a shutter of course.
But I can't help my pursuit of cheap glass haha, thanks!