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  1. #1

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    Preflashing question

    I know preflashing film can control contrast in the negative, but my question is, is there any practical difference between this and just sandwiching an unflashed negative with another one that's been uniformly exposed @ zone 2 or whatever?

  2. #2

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    from my reading and limited experimentation, pre-flashing film can allow you to record more shadow detail, which will not be the same as sandwiching some even density. Someone whith more knowledge and experience can chime in!

  3. #3

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    Quote Originally Posted by Poco
    I know preflashing film can control contrast in the negative, but my question is, is there any practical difference between this and just sandwiching an unflashed negative with another one that's been uniformly exposed @ zone 2 or whatever?

    If I understand your hypothesis, it would depend on whether your "uniformly exposed" second negative was exposed to image bearing or non image bearing light. If you exposed this second negative to image bearing light, the effect would be to increase contrast since the high density regions would receive a a proportionally greater exposure then the shadows. If the second negative exposure was made to non-image bearing light the net effect would be zero because you are adding uniform density equally to all regions of the camera negative density.

    Preflashing film is done to non-image bearing light. The effect is greatest to shadow regions of the camera negative. That would not be the case in your example.

    The proper procedure to reduce contrast using a "sandwich" is to create a positive mask of the camera negative. The net reduction effect of a positive mask would be the peak density of the mask.

  4. #4
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    Quote Originally Posted by Poco
    I know preflashing film can control contrast in the negative, but my question is, is there any practical difference between this and just sandwiching an unflashed negative with another one that's been uniformly exposed @ zone 2 or whatever?
    Flashing the film modifies the silver halide crystals in the emulsion in a way that makes them more sensitive to light (I'm sure there is a rocket-science explanation, but it's not important to anyone except rocket scientists). Flashing the film is generally defined as exposing the film to a small amount of uniform non-image light - such as a diffuse faint lightsource. This increased sensitivity provides greater contrast in the shadow areas, once an exposure is made. Long ago, some photographers would load their film holders in faint moonlight - they probably didn't understand the science behind it, but they knew they got better shadow detail as a result.

    Using a sandwiched negative will not give the same results. If the wandwich in between the shutter and the film, it will just reduce exposure by about 2 stops, it seems to me. If you give 2 stops more exposure to compensate, I think you'd just get the same exposure as without the sandwich.

    -Mike

  5. #5

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    Thanks for the answers.

    Unfortunately I'm still trying to get my head around why there would be a difference between an in-camera preflash and the addition of uniform (non-image) density by way of another negative. The idea that pre-flash somehow sensitizes film seems unlikely since preflash can, in fact, be done after exposure.

  6. #6

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    A pre flash sensitizes the silver halide to the threshold value so that a smaller quantity of light will begin to form an image in the shadow area. You do NOT preflash to the extent it forms a fog. The test is to prefash more and more until it fogs and then back off. You have now given the emulsion as much as you can without changing it.

    The preflash will increase the density in the shadow areas, but has little effect on the highlights as the exposure is so much higher than shadows.

  7. #7
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    A post flash is indeed effective for pretty much the same reason, it is adding energy to the latent shadow detail so it will get over the threshold exposure neccesary to start creating density.
    Gary Beasley

  8. #8

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    Quote Originally Posted by Poco
    ... why there would be a difference between an in-camera
    preflash and the addition of uniform (non-image) density
    by way of another negative.
    That question is not clear; another negative of uniform
    density. That other negative is another negative. Where
    is it to fit in?
    It can in no way affect your scene's recording. A pre-exposure
    will lift the low values placement on the characteristic curve
    in a meaningful way while not affecting the high value's
    placement. If it's not on the film it can't be in the print.
    Pre-exposure puts something on the film. Dan

  9. #9

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    Dan,

    We all know how a fogged negative (from light leak, or whatever) will take a serious hit to contrast in its lower regions. All I'm asking is whether you can achieve the same effect by sandwiching a normal negative with a uniformly fogged one.

    I understand that what I'm suggesting goes beyond traditional pre-flash which doesn't add enough light to create actual density on the negative.

    It's probably one of those questions that only has an experimental answer.

  10. #10

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    Quote Originally Posted by Poco
    I understand that what I'm suggesting goes beyond traditional
    pre-flash which doesn't add enough light to create actual
    density on the negative.
    But that is not the case. I believe I'm correct in
    saying that A. Adams practiced both zone I and II
    pre-exposure. Both those zones are very much
    measurable.

    The purpose of pre-exposure is to lift low values
    higher upon the characteristic curve. Dan

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