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  1. #1
    BetterSense's Avatar
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    flashing and chemical hypersensitization

    I have heard that preflashing to an amount just below that needed to create density will increase shadow speed. I use the technique myself in printing, but I'm not sure it's actually making the paper faster. How long does this effect last? Can I preflash a minute ahead of time, or an hour or day ahead of time? Does post flashing do the same thing?

    What other treatments are there to increase the real shadow speed? I have heard that hydrogen does something; also, baking film?
    f/22 and be there.

  2. #2
    Bob Carnie's Avatar
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    I have pre flashed and post flashed and they seem to be the same

    I do step wedges of time (strip tests) I then pick the flash time of the unit of light just before the one I see tone with.

    pretty simple.. keep the head high on the second enlarger and out of focus, making sure that you have tons of coverage.

    If I was doing a lot of large contact printing, I would make a digital negative to size for the highlight ++++ regions and flash only into them.

    for enlarger printing I just preflash the whole sheet of paper.

  3. #3

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    Quote Originally Posted by BetterSense View Post
    I have heard that preflashing to an amount just below that needed to create density will increase shadow speed. I use the technique myself in printing, but I'm not sure it's actually making the paper faster. How long does this effect last? Can I preflash a minute ahead of time, or an hour or day ahead of time? Does post flashing do the same thing?

    What other treatments are there to increase the real shadow speed? I have heard that hydrogen does something; also, baking film?
    Like Bob says, post- and pre-flashing are essentially the same. The technique works for both film and paper. For paper, the highlights will be affected, for film, the shadows, but both are the low-density areas. In either case, you are not changing film or paper speed at all, but giving an overall exposure that is below the minimum exposure to make a density change in the material.

    What happens is that the film/paper you flash gets enough exposure to reach just below the threshold where the next few photons will make a visible difference in the image.

    For paper, this means that highlights that would otherwise print white now have a little "substance" to them. Keep in mind, however, that the flash exposure will be added to all densities. This makes little difference in mid- to dark tones, but will reduce highlight contrast a bit (to quite a bit, depending on your flash exposure) and may not be the best solution to your particular problem.

    For film, the corresponding thing happens to the shadows. Shadows that would be below the threshold without flashing now have some density on the negative, but the contrast in the shadow area is reduced and shadow separation is not as great. Again, the subject you are dealing with should determine if this is a good solution. I prefer to expose more to get shadow detail instead of flashing with B&W negative materials just because of the loss of separation. For reversal films, flashing is a useful tool to tame highlights. This latter, for me, is the only real reason to flash film.

    I've flashed paper up to days ahead of time and had good results. Usually, though, I flash a test strip for the print I'm working on, and after arriving at the right flash exposure, will flash several sheets of paper and keep them in a paper safe. These I use through the last stages of refining and printing up a run of prints. Flashing minutes or even an hour ahead of time will be just fine. I've always flashed film just prior to exposure with a grey card or the like, but one could, theoretically, flash film in advance.

    If you are searching for more "shadow speed," then you really need faster film... There are some techniques for super-sensitizing film, but they are usually limited to astronomy/scientific photography and are not really practical for the field. Sure, some developers give you more effective speed than others, but this is usually less than a stop. The real speed of an emulsion is in the design.

    Best,

    Doremus

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    It seems to me using lenses with a little more internal flair would produce similar (not quite identical) results.

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    I wonder if pre-flashing film would help in high contrast situations such as church interiors where the overall scene is dark but the windows become too dense? "Way Beyond Monochrome" has a chapter on pre-exposure (ie, pre-flashing) in which they recommend use of white-balance filter.
    van Huyck Photo
    "Progress is only a direction, and it's often the wrong direction"

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    BetterSense's Avatar
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    I know I mentioned paper, but in this thread I wanted to talk about film. I'm thinking about making a device to facilitate preflashing for night exposures. But I don't know if it will work if I preflash before I go out or if I have to flash during the exposure.
    f/22 and be there.

  7. #7
    MattKing's Avatar
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    Quote Originally Posted by BetterSense View Post
    I know I mentioned paper, but in this thread I wanted to talk about film. I'm thinking about making a device to facilitate preflashing for night exposures. But I don't know if it will work if I preflash before I go out or if I have to flash during the exposure.
    Pre-flash or post-flash, it shouldn't matter when you do it.
    Matt

    “Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

    Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2

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    Quote Originally Posted by BetterSense View Post
    I know I mentioned paper, but in this thread I wanted to talk about film. I'm thinking about making a device to facilitate preflashing for night exposures. But I don't know if it will work if I preflash before I go out or if I have to flash during the exposure.
    Hi, I don't know how long the flashed-film effect will hold, but something I've done on occasion is to use a double exposure for the "flash". Just about any constant tone object will work - just put it far enough out of focus. Something like 4 or 5 stops below a metered exposure ought to be in the general range you want. If you have a scene where the shadows just barely go too dark, this works pretty good, but you might have to try a couple variations on the "flash."

    I think, like Old-N-Feeble suggests, that the effect could be made nearly identical to lens flare when the flash exposure is just so. But with a separate "flash" exposure, you can add any amount of effective flare that you want.

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    Quote Originally Posted by doughowk View Post
    I wonder if pre-flashing film would help in high contrast situations such as church interiors where the overall scene is dark but the windows become too dense? "Way Beyond Monochrome" has a chapter on pre-exposure (ie, pre-flashing) in which they recommend use of white-balance filter.
    I would say that pre-flashing might help negative range some small amount in a subject with such light range as that, but it will not render in a print. A situation like that calls for pyro, masking, or dodge & burn. The flashing examples I've seen were more useful on extreme film speed pushing than range of brightness. Churches can have some mighty dark areas even at noon on Sunday, but the windows are a hundred f/stops brighter.

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    Rudeofus's Avatar
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    Athiril (who mostly post in the color section here) has done some incredible work with preflashing, and claims to have gotten quite a bit of extra shadow detail.
    Trying to be the best of whatever I am, even if what I am is no good.

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