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  1. #1
    Michael Slade's Avatar
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    Homemade lens - determining aperture

    Hey all,

    I've built a view camera lens out of an old magnifying glass and want to determine the aperture.

    Where is the formula for determining aperture?

    Thanks!
    Michael Slade

  2. #2

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    It's the focal length divided by the effective lens diameter.
    Jason Brunner has a good video to illustrate; http://www.jasonbrunner.com/videos.html
    Last edited by bdial; 10-22-2008 at 05:11 PM. Click to view previous post history. Reason: spelling

  3. #3
    Michael Slade's Avatar
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    Thanks!!! I knew APUG would come to the rescue!
    Michael Slade

  4. #4
    Chazzy's Avatar
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    You're going to need some tortellini.
    Charles Hohenstein

  5. #5
    Joe VanCleave's Avatar
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    One thing I did recently to my homebrew nested box camera (5" x 7" format) was to use "waterhouse stops" (homemade insertable discs of premeasured openings) for the aperture stop, inserted just behind the lens inside the box. I rate each aperture with its diameter in millimeters, rather than an "f" number. The reason for doing this is that I can then measure the bellows extension of the box, in millimeters, after focussing on the subject and then simply divide this by the diameter of the aperture stop in use to get a direct calculation of the "true" focal ratio, already taking into account the bellows extension factor. Using apertures rated by their diameter thus makes life much easier if you're focussing close-up on objects.

    I carry an el-cheap plastic millimeter scale for use in measuring focal distance on the box.

    BTW, my camera's main lens is a plastic, credit-card sized fresnel magnifier. It's not a Holga, but it has large format Holgaesqueness. Or Holgiosity.

    ~Joe
    Last edited by Joe VanCleave; 10-23-2008 at 11:00 AM. Click to view previous post history. Reason: Writing is 90% editing

  6. #6
    bobwysiwyg's Avatar
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    Joe,

    Interesting. Can you post a pic or two taken with this setup?
    WYSIWYG - At least that's my goal.

    Portfolio-http://apug.org/forums/portfolios.php?u=25518

  7. #7
    Joe VanCleave's Avatar
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    Quote Originally Posted by bobwysiwyg View Post
    Joe,

    Interesting. Can you post a pic or two taken with this setup?
    Here's a still life from my F295 posting, shot in my living room with afternoon sun streaming in through the window blinds. Preflashed grade 2 paper negative, f/18 waterhouse stop, 2 second exposure.

    ~Joe


  8. #8
    Joe VanCleave's Avatar
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    And this shot of the Sandia Mountains, east of Albuquerque, a 7 second exposure with the fresnel lens stopped down to around f/55 with a waterhouse. Preflashed grade 2 paper negative.

    The negatives are held in a homemade black foamcore film holder, using corner mounts, giving the images a pseudo-19th century plate camera appearance.

    ~Joe

  9. #9
    Joe VanCleave's Avatar
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    And this still-life, in my living room with indirect afternoon light. The fresnel lens is operating "wide open" at around f/3.5, so you can see the lovely off-axis artifacts. Preflashed grade 2 paper negative, 25 second exposure.

    ~Joe

  10. #10
    bobwysiwyg's Avatar
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    Very nice, thank you.
    WYSIWYG - At least that's my goal.

    Portfolio-http://apug.org/forums/portfolios.php?u=25518

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