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Thread: Using flash gun to expose paper negatives

  1. #1

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    Using flash gun to expose paper negatives

    Iím trying to get a feel for using a flash gun to expose paper negatives.
    Rather than trial and error Iím initially using logic to give me a start point!!!!
    Therefore, assuming:-
    1] the stated GN of the flashgun is correct and repeatable
    2] the ISO value adopted for the paper used for the paper negative is correct and constant.
    3] the flashgun GN decreases by the square root of 2 for each halving of the ISO number. (ie GN 33 for 100 ISO becomes GN24 for iSO 50 etc Ė light falls off at root2 times the increase in distance)
    Therefore assuming unity for the (any flash gun) GN at 100 ISO, then the correcting factors are:
    0.707, 0.500, 0.354, 0.250 and 0.176 for ISO value of 50 down to 3.
    So for a GN of 48 [meters @ 100 ISO], the corrected GN for 3 ISO is 9 which gives 1.6m @ f5.6
    Is my logic correct or am I missing something that effects the practice of using a flashgun for paper negatives?????
    Regards
    Tony
    PS Mod, move if in wrong forum

  2. #2
    Jac@stafford.net's Avatar
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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by tonyowen View Post
    I’m trying to get a feel for using a flash gun to expose paper negatives.
    Rather than trial and error I’m initially using logic to give me a start point!!!!
    Therefore, assuming:-

    2] the ISO value adopted for the paper used for the paper negative is correct and constant.
    Paper does not respond cumulatively to a number of short exposures.

  3. #3
    Randy Moe's Avatar
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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Jac@stafford.net View Post
    Paper does not respond cumulatively to a number of short exposures.
    Why is that Jac?

    I have used multiple flash strobe to expose X-Ray using pinhole as lens in studio.

    Maybe apples and oranges, but pinhole is slow...

  4. #4

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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Jac@stafford.net View Post
    Paper does not respond cumulatively to a number of short exposures.
    My question did not refer to multiple exposures, I'm only considering a single exposure.
    regards
    Tony

  5. #5

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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Jac@stafford.net View Post
    Paper does not respond cumulatively to a number of short exposures.
    Sure it does. Remember Fred Picker and his cumulative 3-second bursts of light printing technique? Heck I switch my enlarger lamp on and off, cover and uncover parts of the print and sometimes flash a print hours before I make the negative exposure; everything comes out just fine. The Beseler-Minolta computer head used a rapidly pulsing light source IIRC.

    Now, whether the short duration of the flash will be in the reciprocity area of the paper is a different issue, but I see no reason not to try multiple pops and see how it works. If there is reciprocity failure, the flash exposures will accumulate, just not linearly.

    Best,

    Doremus

  6. #6
    Randy Moe's Avatar
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    Re: Using flash gun to expose paper negatives

    Let us know how it develops.

  7. #7

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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Doremus Scudder View Post
    Heck I switch my enlarger lamp on and off, cover and uncover parts of the print and sometimes flash a print hours before I make the negative exposure;
    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

  8. #8
    multi format
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    Re: Using flash gun to expose paper negatives

    hi tony

    vc paper's iso is between 6ish and 24 depending on the paper
    and the light being used. and it will be slower if you use a yellow (contrast) filter

    have fun!
    john

  9. #9

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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by Randy Moe View Post
    Why is that Jac?

    I have used multiple flash strobe to expose X-Ray using pinhole as lens in studio.

    Maybe apples and oranges, but pinhole is slow...
    Look up "Claydon effect" in an old photo reference... This is why multiple flash pops require more and more pops to progressively build up more exposure...

    For the OP, allowing more time with continuous light might be easier than a small flashgun...

    Steve K

  10. #10

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    Re: Using flash gun to expose paper negatives

    Quote Originally Posted by LabRat View Post
    For the OP, allowing more time with continuous light might be easier than a small flashgun...Steve K
    Sorry people, I'll rephrase my question:
    All things being equal, the GN of any flash gun vary with the ISO number of the film/paper being (singularly) exposed. Also, the (normal) series of ISOs from 400 down to 3 halves or doubles for two adjacent numbers (ie 100 to 200 ISO doubles, whereas 100 to 50 ISO halves).
    Consequently are my 'correcting factors' valid - as a both as a theory and for use in practice?
    Or for very low (sub 25 ISO) paper/film exposures just something else have to be considered for (flashgun) exposures of around 1/60th sec?
    I'm thinking of something similar to the correcting factors used for (say) bellows extension of very long exposures >10s
    regards
    Tony

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