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Thread: Using flash gun to expose paper negatives

  1. #11

    Join Date
    Sep 1998
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    Oregon and Austria
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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by tonyowen View Post
    Have I confused people - My question referred to the taking of an image using (say) multigrade paper.
    I'm not referring to pre-flashing of paper before an exposure.
    Simply using a flashgun as on film except I'm using paper with a low ISO rating.
    regards
    Tony
    Tony,

    I'm not confused, just responding to something else than your original question. Totally off-topic; my apologies

    To answer you: Aperture scales and flash guide numbers are based on the inverse-square law. Since whole-stop apertures correspond to doubling or halving the ISO, you can therefore use the same factor to calculate flash distance for a given ISO instead of a given aperture; the factor, the square root of 2, is the same.

    You can simply repeatedly divide the GN by the square root of 2 to get a distance for an ISO sequence in whole stops, or, as you have done, use the reciprocal values as multipliers. (1 / square root of 2 = 0.707 approx., etc.). As far as I can see, your calculations are correct.

    The thing that may affect flash exposure for your paper is the short duration of the flash itself. I'm not sure there won't be some reciprocity failure. Most people know reciprocity failure only for long exposures. The same thing happens with very short exposures as well. Films are designed to be exposed by flash, with exposures sometimes shorter than 1/1000 of a second. I don't know about paper emulsions. You may have to add exposure if the short exposure of the flash does, indeed, elicit reciprocity failure. Even if you are setting your shutter to 1/60 second or whatever, it is the actual duration of the flash that determines length of exposure. Tests will tell the tale.

    And, don't be afraid to use multiple pops if you find it helps

    Best,

    Doremus

  2. #12

    Join Date
    Jun 2015
    Location
    Anglesey, North Wales, UK
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    256

    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Doremus Scudder View Post
    Tony,As far as I can see, your calculations are correct. You may have to add exposure if the short exposure of the flash does, indeed, elicit reciprocity failure. Even if you are setting your shutter to 1/60 second or whatever, it is the actual duration of the flash that determines length of exposure. Tests will tell the tale. And, don't be afraid to use multiple pops if you find it helps Best, Doremus
    Thanks Doremus - no need to apologise.
    So, my maths are correct and I have a starting point given my assumption of paper ISO.
    Therefore all things being equal - If my "natural light" paper negatives' are okay; and the "flashgun" paper negatives are okay then my assumptions become facts.
    If the flashgun paper negatives are NOT okay then I need to apply some fudge factor when using a flash gun to expose a paper negative.
    regards and thanks again
    Tony

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