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Thread: Figuring Aperture Scale, Missing on Dagor

  1. #1

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    Figuring Aperture Scale, Missing on Dagor

    Hi,
    I'm ready to try a tiny barrel f6.8 Dagor, but the aperture has no scale. Is there a way to get a close swag on making the hole different f sizes?

    I shoot wetplate, so wide open at f6.8 will work. But I hear these Dagors are so sharp, thought I'd try it at some tighter apertures. I could just close it "halfway" and "mostway" and guess....

    The lens was discussed here, with a pic.
    http://www.largeformatphotography.in...ad.php?t=30984

    Thanks,

    Garrett

  2. #2

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    Re: Figuring Aperture Scale, Missing on Dagor

    There is definitely an equation to work it out, since you know aperture and focal length.

    I can't remember what it is, but it's to do with the apparent aperture (as viewed looking at the lens) when closed down as a fraction of the maximum and the focal length. No idea off the top of my head, but I'll get back to you if I can.

  3. #3
    Moderator Ralph Barker's Avatar
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    Re: Figuring Aperture Scale, Missing on Dagor

    If you pre-calculate the standard f-stops for the focal length, you can then measure the apparent aperture looking through the front of the lens, and mark a scale made from tape or other material. You can use a rule marked in millimeters, or use a dial caliper, which is probably more accurate. Using only one eye helps, too.

    For example, f/8 = 1/8 = 0.125 x 120mm = 15mm aperture opening

  4. #4

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    Re: Figuring Aperture Scale, Missing on Dagor

    Two weeks ago in another thread was an explanation to calculate the f-stops http://www.largeformatphotography.in...ad.php?t=32332
    This method works with long focal lengths.

    Peter K

  5. #5
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Figuring Aperture Scale, Missing on Dagor

    Adjust the aperture untill it's almost but not quite visible through the front of the lens. Then unscrew (at least) one of the lens cells. Measure the aperture opening. Set a mark on the aperture scale. Close the aperture until the opening is exactly half of what it was. Set another mark, that's two stops down from wide open (f:13.6).

    Repeat as necessary.

  6. #6

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    Re: Figuring Aperture Scale, Missing on Dagor

    These are all good, helpful plans. I thank you. I'm going to the shop now to make a lensboard for the speed graphic. I'll set up some aperatures, and try it tomorrow!

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