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Thread: Aperture in MM? Help!

  1. #1
    ScottPhotoCo's Avatar
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    Aperture in MM? Help!

    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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  2. #2

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    Re: Aperture in MM? Help!

    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.

  3. #3

    Join Date
    Nov 2007
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    1,127

    Re: Aperture in MM? Help!

    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.

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