Offset aperture opening

[rated]

Can anyone assist in calculating the aperture when the opening through which the image will pass is offset from the rear of the lens itself, as depicted in the attached picture.

Opening through which image is projected is 10mm smaller than the rear element (83mm), and 100mm from the centre of the lens, or 40mm from the rear lens element. Lens is 400m f/4

Any help appreciated.

[ridax]

I am afraid there is not enough data for the calculations. What you need is the size of the aperture magnified by the lens, as it looks through the lens from the lens front. As the lens magnifying power for that aperture to the lens distance is not known, it is easier to measure the effective aperture than to calculate it. Strictly speaking, you need collimated light to emulate looking from infinity to make the measurements. But a good enough an approximation is to look at the lens front from a long enough distance, as long as you can see from (binoculars may help) with a ruler put in front of the lens.

That said, in your case the difference between the glass and the aperture sizes is small enough so you may have just the same f/4 with the aperture as without it (magnified by the lens, that aperture may just get to be the same or bigger then the lens elements when seen from the lens front). But with such an aperture, there certainly will be a lot of vignetting. So you may get the full f/4 at the image center but rapidly darkening corners. That makes the exact aperture calculations/measurement not worth it at all. The actual exposure will differ too much across the covered field.

[ic-racer]

I think you are mistaking 'aperture' and 'vignetting.' The lens mount in the diagram posted will produce vignetting not a mathematical change in aperture.

[ridax]

I think you are mistaking 'aperture' and 'vignetting.' The lens mount in the diagram posted will produce vignetting not a mathematical change in aperture.

No that is not a mistake. Both the vignetting will be increased and the aperture will get smaller - if the aperture seen from the front of the lens is smaller than the lens opening without that aperture.

The 135mm tessar-type Mamiya TLR lens has all its glass in front of the shutter so its aperture is far enough behind the lens itself. The aperture works just like any other aperture should (of course that lens was constructed to have its aperture in that place from the start - that's why it works).

I bought a 365mm f/3.65 Soviet projection triplet half a year ago (just out of curiosity). I did not like its image wide open, and being made for projection, the triplet did not have an iris. But luckily its most narrow part was the rear end of the barrel so I did not need to disassemble the lens to put an aperture inside it. I just put card apertures behind the rear element and tested the lens performance. That was really easy to do. And the triplet turned out to perform beautifully at f/5 and smaller stops.