determining aperture of hacked lens

Discussion in 'Camera Building, Repairs & Modification' started by rippo, Feb 7, 2008.

  1. rippo

    rippo Member

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    I had an Olympus OM 50mm fail on me awhile back, and decided just now to pull off some elements and see if anything would be usable on my 4x5 (a weird little hobby of mine, I know).

    I removed the front two elements of this lens, which are cemented together in a mount, and I have no desire to remove them. I first tested the lens the right way round, and got an approximate focal length of 100-110mm or so (translucent oatmeal container lid at one end of a ruler, lens along the ruler, pointed out the window, until infinity comes into focus. quick and dirty!). Sort of a workable focal length, but a little tight (i.e. short)with my packard shutter mounting system.

    Then I turned the lens around and was surprised to find that I had a completely different focal length. Something around 230mm or so! I had been expecting it to be symmetrical, but it's not.

    So how do I determine the aperture for this lens? Do I just measure the diameter of the entrance pupil? There's no internal aperture to this. The different between the front and rear elements is substantial, roughly 28mm vs 20mm. I keep thinking to myself, hey I want the aperture of the 28mm side!

    So is that it? diameter of the element facing forward is the aperture, end of story? There are no internal rings or anything that limit the diameter further.
     
  2. Steve Smith

    Steve Smith Member

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    Aperture = focal length / aperture diameter. In this case if you don't have a variable aperture, just use the cell diameter. Should be close enough.


    Steve.
     
  3. rippo

    rippo Member

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    thanks steve. what i'm trying to do though is figure out the aperture diameter. is it really that i use the lens diameter of the element facing the subject, even though the diameters of the two elements are so different?

    attached are some pictures of the same lens, front and back.

    [​IMG]
     
  4. keithwms

    keithwms Member

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    Use the most limiting diameter.
     
  5. ic-racer

    ic-racer Member

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    Yes. Use the front facing view. If in doubt, check some of your other lenses that have aperture scales and you will see.