Pre-exposure

Discussion in 'B&W: Film, Paper, Chemistry' started by JackRosa, Dec 10, 2004.

  1. JackRosa

    JackRosa Member

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    I have been pre-exposing my B&W negatives for quite some time for Zone II. Any members with experience on this technique who could contribute insight?
     
  2. matt miller

    matt miller Subscriber

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    Are you pre-exposing all of your negatives, or just the ones to be used in high SBR situations? I'm not too familiar with the reasons for pre-exposure. I always thought it was a tool to be used for high contrast subjects. How are you currently doing the pre-exposure?
     
  3. Donald Miller

    Donald Miller Member

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    I have used this technique because it has the opposite effect of preflashing the paper at the printing stage. In other words it compresses the shadows rather then compressing highlights.

    I have exposed extreme SBR scenes as high a Zone IV on the pre-exposure and then compensated my actual exposure by basing it on the highlight values.
     
  4. Peter Schrager

    Peter Schrager Subscriber

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    Bleaching

    Jack-Alot easier to make up a dilute soluton of Ptt. Ferricyanide and PAINT away the black or lighten the highlites. Not all papers respond to this. Forte Poly V is the one I use and I learned the technique out of Bruce Barnbaums book.
    Regards Peter
     
  5. Donald Miller

    Donald Miller Member

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    Actually bleaching paper will increase print contrast...exactly opposite of what wants to accomplish when a negative of too great contrast exists. Preflashing paper will decrease print contrast by compressing highlights. Pre-exposing film will decrease contrast by compressing shadows.
     
  6. JackRosa

    JackRosa Member

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    To: Donald

    Compressing shadows? By pre-exposing, don't you move the low areas up in the curve, away from the toe? Wouldn't this actually increase separation in the shadows?
     
  7. dancqu

    dancqu Member

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    I think an important point is being overlooked in this discussion.
    If one is seeking to move the low areas up the curve and thereby
    increase the separation, give the film an extra stop or two of
    exposure.

    A pre-exposure will do the same only at the toe. The scene be
    a very contrasty one, it will have the advantage. A Zone I
    pre-exposure I'd think always safe while a Zone II might
    be pushing it. I've the negative's density in mind.

    Also, I do not think has been mentioned the effective increase
    of the EI due to pre-exposure. Dan
     
  8. JackRosa

    JackRosa Member

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    Dan - excellent points, re: over-exposing and Zone I pre-exposure. Thanks.
     
  9. dancqu

    dancqu Member

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    Put the paper's and the film's toes, little exposed areas, on the
    left of the horizontal x axis of an xy graph. Put the density on the
    vertical y axis. Exposure increases towards the right along the
    x axis.

    Assuming both film and paper have very non-linear toe regions
    then the effect of pre-exposure should be similar. For both film and
    paper the for real exposure lies above the pre-exposure and the
    whole of exposure, hopefully, resides within the more linear
    portion of both curves.

    That's the way I see it. If I've overlooked some fact or reasoned
    wrong let me know. Dan
     
  10. Donald Miller

    Donald Miller Member

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    Nope what you are doing is adding a greater proportion of exposure to the lower zones then to the higher zones this will compress the low values. What you are addressing is increasing linear exposure. Pre exposure is non linear.
     
  11. JackRosa

    JackRosa Member

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    To: Donald

    Roger, roger on that. Thanks.
     
  12. dancqu

    dancqu Member

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    Yes, I did address the matter of linear and non-linear in my
    post just previous your last.

    Howard Bond agrees with you; compression in the lower zones.
    But Howard in graphing of the curves, pre vs no pre-exposure
    has moved the pre-exp. curve speed point one zone to the
    left, ie he has doubled the speed of the film or paper.

    To put it another way his pre-exp. start of toe is at half the
    EI as his start of toe for the post pre.; the for real exposure.
    In a previous post this thread I did mention the increase in
    EI when using pre-exp.

    In effect a pre-exp. of zone 1 places the post zone 1 exp. at
    a zone 2 density. That density is towards the more linear
    portion of the curve. Mr. Bond sees a compression which
    is not really there. Dan
     
  13. Donald Miller

    Donald Miller Member

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    I recognize what you are saying about the effect of preexposure reducing the EI. While that is true for the lower zones it does not translate linearly to the upper zone exposures.

    It's interesting since what I am about to state is what Howard told me at a workshop almost twenty years ago.

    If we recognize that each stop of increased exposure amounts to a doubling of light then we can readily assign a numerical value to those stops for the purpose of illustrating the effects of pre exposure to non image bearing light This is what I have done in the following.

    The pre- exposure in this example is at a zone I luminance to non image bearing light. I doubt that the post will have the columns neatly aligned as I have composed them. I would ask that you sort them out in a understandable format.


    Zone Numerical value Pre-exposure New Value

    I 1 1 2

    II 2 1 3

    III 4 1 5

    IV 8 1 9

    V 16 1 17

    VI 32 1 33

    VII 64 1 65

    VIII 128 1 129

    As one can see the relative effects of pre-exposure are greatest in the lower zones and virtually non existent in the upper zones. My negatives bear this out.
     
  14. dancqu

    dancqu Member

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    You've got that bassackwards. Pre-exposure increases the EI
    as one might expect and as H. Bond graphicly depicts. Dan
     
  15. dancqu

    dancqu Member

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    That last post of mine may need some explanation.
    The film or paper does not magicaly become more fast.
    The effective increase in speed is due to the pre-exposure.

    I'd think pre-exposure most usefull with very high contrast
    scenes or when for some reason an extra stop in film speed
    would be nice to have. Otherwise give the film an extra
    stop or two. That alone will lift the shadows into a
    more linear portion of the curve. Dan
     
  16. Kirk Keyes

    Kirk Keyes Member

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    Notice in the table that Dan made three posts up, the data he gives for a fogging pre-exposure of the film is exactly the same as that which would be seen from the effects of lens flare.

    Look in a copy of "Basic Techniques of Photography" by Ansel Adams, John Paul Schaeffer, in the first edition (I'm don't have the revised edition so it may be different), Schaeffer has two examples that demonstrate the use of preexposure.

    One example is to reduce the contrast of a high contrast scene by using the fog to compress the density range in the low values - see Dan's chart above. Same effects as seen from lens flare.

    The other example he shows for using a fogging preexposure is for use with low contrast scenes. I believe this example used a greater exposure than the previous one and was used to give a slight increase in tonal separation by pushing the lower zones much higher up onto the film curve so that they were no longer on the toe area. I suspect this was more benificial on films that have long toes, like Tri-X.

    So I believe, that based on the amount of pre-exposure that is given, the type of film, and the original scene contrast, this technique could be used for two different effects.
     
  17. Donald Miller

    Donald Miller Member

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    I have actually preexposed as high as a Zone IV and then exposed for highlights with normal development.
     
  18. ChuckP

    ChuckP Subscriber

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    For low contrast scenes why not just give more exposure to get off the film toe? This gets you up on the straight line portion. And without the contrast reduction in the shadows that results from pre-exposure.
     
  19. esearing

    esearing Subscriber

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    Ansel has a chapter dedicated to this subject in "the Negative" (Filters and PreExposure). He explains the lighting/contrast conditions where this is most appropriate and compares the process results to overexposure.