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Hello all,
I just received a Carl Zeiss, Jena marked No. 276052 Tessar 1:4,5 F~ 21CM DRP IIA. It is in beautiful shape but it looks like the f-stops are marked as millimeters. I've never seen this before. The scale seems to run from 4 to 39 with 4 being the smallest opening and 39 the largest. Have any of you seen this before? What is the easiest way to determine the more standard f-stop for ease of use?
Thanks in advance!
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Zeiss did this for a while. Some macro lenses from microscope manufacturers also have apertures in mm.
You can get close enough to f/numbers by dividing focal length in mm by aperture in mm. 210/4 gets you f/52.5, 210/39 means f/5.4, and so on. Not exact because there's no guarantee that the entrance pupil will be the same size as the aperture, but close enough.
You can alway try corroborating what you get from Dan's formula with what you
calculate if you were to make waterhouse stops for your lens.