Preflashing question

Discussion in 'B&W: Film, Paper, Chemistry' started by Poco, Oct 12, 2005.

  1. Poco

    Poco Member

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    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. Nige

    Nige Subscriber

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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. Donald Miller

    Donald Miller Member

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    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. mikewhi

    mikewhi Restricted Access

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    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. Poco

    Poco Member

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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. Ronald Moravec

    Ronald Moravec Member

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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. glbeas

    glbeas Member

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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.
     
  8. dancqu

    dancqu Member

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    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. Poco

    Poco Member

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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. dancqu

    dancqu Member

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    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
     
  11. Donald Miller

    Donald Miller Member

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    I have preflashed film on Zone III and IV at times when I have had extreme brightness ratio situations. (14 zones).

    Preflashing at that high a level will compress the high brightness ratios to a point where I can print the negatives with no further corrective actions at the printing stage.

    Preflashing compresses shadow values whereas flashing of paper at the printing stage compresses highlights. I prefer to compress shadows because most people expect shadows to have less tonal separation.

    If one were to uniformly expose a second negative at that high a level and sandwiched it with the camera negative at the printing state it would manifest itself as uniform density. Preflashing is proportional in it's effect.
     
  12. Jim Noel

    Jim Noel Member

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    Pre-exposure

    Pre-flashing film is an old technique used by news photographers in the days of the Speed Graphic. By exposing the film to an amount of light equal to Zone I or II any additional light, howver slight, will record. This in effect adds exposure in the shadows. This small amount of light has a very minimal effect on the highlights.

    At one time this was said to increase the film speed. It does not do so, it merely adds exposure where it is needed most - in the shadows.

    Today I frequently use it when photographing against the light to open up the shadows without blowing out the highlights.
     
  13. Poco

    Poco Member

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    "Preflashing is proportional in it's effect."

    That's the part of this whole thing I don't understand, Donald. Why wouldn't the sandwiched neg also be proportional? If you preflash film at higher zones, say zone 3, that exposure is added to the highlights as well, though it represents a proportionally lower addition to their densities. Why wouldn't a sandwiched negative also not add to their densities at a proportionally lower level?

    I don't mean to beat this horse, it's just that I find it an interesting question and would like to understand the theory.
     
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  15. Donald Miller

    Donald Miller Member

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    Poco,

    I have given this more thought and I see where you are approaching this from. I also understand the basis of your question. You pose an interesting question and I had never considered this before. It appears that when I apply mathmatical values to the two methods that the effect should theoretically be the same, now that I think about it more.

    If someone were to uniformally expose several negs at differing low density values these could be used in the darkroom as effectively as pre-flashing the film at time of the exposure provided the conditions that I next mention have been satisfied as well.

    The thing that may be lacking in your approach would be the potential for inadequate low value camera negative exposures. Additionaly, if one were to adequately expose shadows, in high brightness ratio situations, then the potential exists for placing the highlight values on the shoulder of the film's characteristic curve. Both of these potential problems would be eliminated by preflashing the film in camera at time of exposure.

    For instance, in high brightness ratio situations, when I pre-flash film at Zone III or Zone IV values, I will make my exposure determination for the second exposure based upon highlight value placements and not meter readings based on shadow values.

    I think that an unsharp mask would provide the same effect of reduction of density range and additionally provide enhanced apparent sharpness due to edge effects...although an unsharp mask would necessitate making a mask on a case by case basis.

    Have you tried the approach that you have suggest? I would be interested to learn about your results. Thanks...
     
  16. Poco

    Poco Member

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    "differing low density values these could be used in the darkroom "

    That was my idea....

    "the potential exists for placing the highlight values on the shoulder of the film's characteristic curve"

    ....and that was one of my concerns, at least initially. But shoulders are a function of a *single* negative's response to high exposure and since the additional "exposure" (density, really) in this case would be by way of the sandwich negative, it should simply be additive without shouldering out. How the paper handles it is another matter.

    It really is something to fool with experimentally in the coming cold weather.
     
  17. Neal

    Neal Subscriber

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  18. dancqu

    dancqu Member

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    I'll make it easy. Use "differing low density" ND filters
    within the enlarger or under the lens. If you have positive
    results, film pre-exposure can be dropped in favor of dial
    it in when the printing is to be done. Dan
     
  19. glbeas

    glbeas Member

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    It's not the density that does the trick, you are confusing cause and effect. Its the added light to the shadows that bring out the detail inherent. Film has a threshold level below which exposure generates no density. The flash exposure combines with the image exposure to jump that threshold and make image detail where none was. Adding film density after the film is developed is pretty much wasted effort, as you haven't got that shadow detail to do anything with.
     
  20. Donald Miller

    Donald Miller Member

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    I agree with your assessment about bringing the exposure to a threshold...however I disagree with your position about additional density accomplishing nothing. When I preflash at a zone III or IV value for a scene brightness ratio of 14 stops then the additional density beyond a "trigger exposure" serves to compress the 14 stop range.

    Additionally if the film is properly rated then a preflash at a Zone I or II exposure value will be represented as density on the camera negative.

    Theoretically, taking Poco's hypothesis, it would make no difference if the additional density beyond a "trigger exposure" was added in preflash of the camera negative or in a second neutral density negative added to the camera negative and printed as a sandwich of the two. The effect would be to compress the density range of the negative.
     
  21. davet

    davet Member

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    I have given this more thought and I see where you are approaching this from. I also understand the basis of your question. You pose an interesting question and I had never considered this before.

    No doubt I'm being thick here, but I do not. I understand "classic" preflashing in the sense of filling in the toe, but what does the original poster mean by "sandwiching"? Is this at printing, in camera (pretty snug filmholder, that)? What is the reasoning behind it?
     
  22. Donald Miller

    Donald Miller Member

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    What Poco is proposing is that if one exposed a negative(s) to a uniform non image bearing light at a variety of exposures and resultant densities that these could be sandwiched with the camera negative at printing and the effect would be the same as pre-flashing a camera negative provided the camera negative shadow exposure was sufficient and the highlights densities were not lumped on the shoulder.

    The reasoning behind this is that the density range would be compressed at the printing stage in exactly the same way that preflashing does at the film exposure stage.
     
  23. dancqu

    dancqu Member

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    [QUOTES=davet]
    "No doubt I'm being thick here, but I do not. I understand
    "classic" preflashing in the sense of filling in the toe,"

    Very full! I've mentioned A. Adams use of Zs 1 & 2 and Donald
    has mentioned using Zs 3 & 4. Perhaps some are thinking of
    paper pre-exposure.

    "what does the original poster mean by "sandwiching"?
    Is this at printing ..."

    My question a few posts back. Yes, "sandwiching" when
    printing.

    "What is the reasoning behind it?"

    The reasoning behind it is faulty. Dan
     
  24. ChuckP

    ChuckP Subscriber

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    When flashing the negative you are adding a quantity of non image forming light to the image light. This will change the tonal relationships (contrast) of the negative more in the low light shadows and hardly at all in the higher light areas. When you sandwich a uniform density piece of film with the negative you are reducing all the light hitting the paper in the same proportion. The negative contrast is not changed. The same effect as just reducing the enlarger light intensity. If you (in a separate exposure) first expose the paper to non image light and then to the image light you will change the tonal relationships on the paper. This will work opposite of negative pre flashing and effect mainly highlights. How does all this sound?
     
  25. Donald Miller

    Donald Miller Member

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    Well, at first glance this sounds exactly like my reasoning before I applied numerical values to the hypothesis that Poco proposed.

    However, when I began to approach this from a factual standpoint my earlier reasoning did not hold up.

    What I finally came to was that if one is preflashing at a Zone I or II value then density is being added to film. It is not factually a matter of simply adding below exposure threshold light.

    As I have mentioned, I sometimes preflash as high as Zone III or IV to compress extreme brightness situations. In those instances the density addition is substantial. After a pre-flash of that great an amount, I calculate my second exposure based on highlight placement and not shadow values.

    In that case, I don't believe that Poco's conjecture would produce the same result. Whereas it may very well produce a similar result with lower pre-flash placements.
     
  26. Poco

    Poco Member

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    "The same effect as just reducing the enlarger light intensity."

    This is what it all boils down to. Would it be the same effect or not? I'm not sure.

    We know that the recommended best practice in developing extremely old, outdated film is to overdevelop for increased contrast to compensate for elevated base fog. Obviously that base fog does much more than just reduce enlarger light intensity. The question is whether "fog" provided by a sandwich negative would function in the same way.

    I'm far from my darkroom or I could have tried it in the time I've spent yammering about this thing.