Originally Posted by
alt.kafka
OK, that makes total sense. So the implication of this is that, for a given aperture, say f/5.6, the diameter of the iris will necessarily be larger or smaller depending on the focal length of the lens. I understand that the actual diameter is a little difficult to measure, due to the variety of configurations of front lenses. But, in general, it should be necessary that some lenses will have a different iris size, and consequently, 'extra' aperture.
So, for the 135mm Symmar and Copal 0, I can accurately measure that the rear diameter is actually 19mm, and my actual iris opening is around 15mm. So, that looks like it's around 2/3 of a stop difference. When I unaccurately measure how these look through the front lens, it comes out to ~24mm, as you said, and the maximum iris opening is around 19mm. i.e. the same ~2/3 stop difference, as one would expect.
So maybe I just need to put a little more aperture on this compared to the scale.
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