Aperture in MM? Help!

[ScottPhotoCo]

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!

142640

142641

[Dan Fromm]

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.

[Jim C.]

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.