A kind help request about... math!

Discussion in 'Camera Building, Repairs & Modification' started by summerbee, Jan 23, 2013.

  1. summerbee

    summerbee Member

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    Hallo dears,
    doing calculation has never been my best skill, so I am here to kindly ask for a little help. I am purchasing a 6*7 graflex 120 magazine, that should be the base for building a pinhole fixed focal length camera.
    I already own a 0.2 mm pinhole, but trying out the usual pinhole calculation tool i was not able to get a focal distance that could give a wide angle of view and enough coverage for the negative.
    I can order a new pinhole if needed, but now i have no idea how to get a corret coverage of the negative and the FOV of a wide angle lens, something like a 24 or 35 mm on 135 format.
    thx for Your attention, have a nice day You all!
    :wink:
     
  2. nicholai

    nicholai Member

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    Which calculator did you use?
    using this one, you can input any number of variables yourselves, and being a 6x7 user myself, i knew you should use a focal length of 50mm. This will provide coverage seeing as the diameter of the image circle is focal length * 1.92, in this case 96mm, and you only need to cover 60 by 70mm.
     
  3. summerbee

    summerbee Member

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    it's the same calculator i used, but messed up a little with the diameter of the pinhole i already own, a 0.2 that won't fit my needs...
    okies, so a 50 mm focal length and a 0.3 pinhole will do. I need another pinhole then! :smile:

    thank you very much, have a nice day
     
  4. nicholai

    nicholai Member

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    You can easily do with a 0.2 pinhole - it will just need a longer exposure time! No problem.
     
  5. Jim Jones

    Jim Jones Subscriber

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    The 0.2mm pinhole should be fine for about 40 to 50mm focal length. Some of us prefer such wider coverage in pinhole photography.
     
  6. summerbee

    summerbee Member

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    agree, me too, my doubt was circle wouldn't cover image frame.

    let's suppose i'd set a focale length of 43/44mm, that is roughly the half of the negative diagonal (87mm), what would be the conseguent viewing angle?

    another dumb question: i know the process to create a "sport viewfinder" (a frame of the correct dimension in the correct place); is there any simple way to build up or to reuse any optical VF?

    thx again you all.
     
  7. Jim Jones

    Jim Jones Subscriber

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    When the focal length is half of the negative diagonal, the viewing angle will be 90 degrees, corner to corner. There will be considerable fall-off in illumination in the corners, but some photographers are happy with even wider coverage in pinhole photography.

    A sport finder of the same size and proportions of the negative image can accurately frame the view if the eye is centered at the focal length behind the finder. Accurately positioning the eye is tricky. Adapting the viewfinder from other cameras may be more difficult; they rarely cover a really wide angle.
     
  8. summerbee

    summerbee Member

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    mmm i don't like that fall off that much... i'd rather loose some degrees and get a more uniform coverage.
    Since i'm unable to figure it out, would it make a big difference between 50mm and 55mm?
    thank you JJ