Help please. Aperture measure.

[basiltahan]

I have a Schneider 75mm f8 lens in a Copal 0 shutter without Aperture markings.

I want to mark it myself.

The aperture has 5 blades, so it is the shape of a pentagon.

When measuring the diameter of the aperture to calculate the f-number, should I measure the inscribed circle, circumscribed circle or average the two?

Thanks.

[Paul Cunningham]

I was just reading a different thread that discusses this issue, but takes a different approach. If you haven't seen it, here it is: http://www.largeformatphotography.info/forum/showthread.php?106657-Checking-calibration-of-aperture-scales

[basiltahan]

Thanks for the reference.

I read the entire thread.

I find it petty and relatively useless. My opinion.

I want a mathematical calculation for f-numbers.

I have a very accurate tool for measuring small distances.

If the aperture were circular, I'd be sorted.

But it is a (curved sided) pentagon.

[Bernard_L]

As far as I can tell from your description "curved sided pentagon", if your take the average of inscribed and circumscribed circle you should be within 1/10 of an f-stop of the "exact" result. Loss to reflections (the difference between F-number and T-number) and corner falloff probably are of comparable magnitude.

One caveat: the f-number is f/D, where D is not the physical diameter of the aperture (as you would measure after dis-assembling the lens), but the entrance pupil. That is the image of the diaphragm, as you see it from the entrance side of the lens. Suggested procedure: mount the lens on a micrometer translation stage, optical axis pointing up (white paper below), translation horizontal. Focus a low-magnification microscope with a reticle on one edge of the entrance pupil. Move translation stage until you sight the opposite edge; deduce diameter of entrance pupil.

[basiltahan]

As far as I can tell from your description "curved sided pentagon", if your take the average of inscribed and circumscribed circle you should be within 1/10 of an f-stop of the "exact" result. Loss to reflections (the difference between F-number and T-number) and corner falloff probably are of comparable magnitude.

One caveat: the f-number is f/D, where D is not the physical diameter of the aperture (as you would measure after dis-assembling the lens), but the entrance pupil. That is the image of the diaphragm, as you see it from the entrance side of the lens. Suggested procedure: mount the lens on a micrometer translation stage, optical axis pointing up (white paper below), translation horizontal. Focus a low-magnification microscope with a reticle on one edge of the entrance pupil. Move translation stage until you sight the opposite edge; deduce diameter of entrance pupil.

THANK YOU!

What would your guess be on how much different (size) the image of the diaphragm be from the physical diameter of the aperture in a 75mm lens? Are we talking less than 1/2 a stop?

[Jim Jones]

THANK YOU!

What would your guess be on how much different (size) the image of the diaphragm be from the physical diameter of the aperture in a 75mm lens? Are we talking less than 1/2 a stop?

It depends on the lens. When comparing the apparent diameter of the aperture of a retrofocus or true telephoto lens from both front and back the difference is conspicuous. Most lenses would not exhibit such a difference, nor would the actual diameter of the diaphragm be greatly different than the entrance or exit pupil. It's the entrance pupil, not the actual diaphragm diameter, that is important in most optical calculations.