Pre-exposure & Reciprocity

Discussion in 'Exposure Discussion' started by Annie, Jan 20, 2004.

  1. Annie

    Annie Member

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    I was just wondering about the correct method when using pre-exposure of film for increased shadow densities that involve exposures that require reciprocity compensation. In particular when the overall exposure time must be increased due to reciprocity but the ZoneII pre-exposure falls outside of the parameters requiring film reciprocity compensation.

    For example let's say I am shooting 100ASA film and get a reading of 9 on my Pentax Spotmeter and wish to use f45 for my exposure. At f45 the exposure time is 4 seconds and a compensation for reciprocity must be made. However, if I wish to employ a pre-exposure at ZoneII the indicated exposure time read from the meter is ½ second, this is outside the parameters of the films reciprocity.

    My question: Will the ½ second pre-exposure be sufficient to bring the increased shadow density or must this be increased by a factor due to the increased overall exposure that will be required because of the reciprocity effect?

    Thanks!
     
  2. cesarb

    cesarb Subscriber

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    Hi, Annie.

    To my understanding, I suppose you shouldn't care about reciprocity while making pre-exposure. This is usually a short exposure at higher values than you find at low zones. Maybe, it can even supress further corrections, as the extended exposure is just supposed to increase density at those critical zones and pre-flash always reduce contrast, a major problem on reciprocity failure. A simple test, with roll film, may show you after a set of exposures, with and without pre-flash, how these variables meet. The fact that different emulsions show different reactions to both actions, makes testing even more appealing.
    I hope this help.
     
  3. Donald Miller

    Donald Miller Member

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    Annie,
    Pre-exposure of film is normally done in situations involving extreme scene brightness ratios. Let's take a situation in which you measure the low values (using your example) of nine on your meter as the Zone II placement and that you then find that your brightest value in the scene falls on a a Zone IX. Let's say that you really want more detail in the shadows then a Zone II value but to do so would increase the high value placement even more. Since you are in reciprocity with it's attending increased contrast conditions you decide against placing your low values higher in the exposure.

    What a Zone II pre-exposure will do is bump your Zone II scene shadow to a Zone III density on your film. Pre exposure will not work without using diffusion-by this I mean that you must meter through the diffusion and you must pre-expose through the diffusion.

    What we want in pre-exposure is non-image bearing light. When you then give a normal exposure and develop your negative normally it will then give you a density range of III to IX. However in printing the negative you can have this represented as a print tonal value of II to VIII since film will record more luminance then paper.

    Now to your question. If you metered the scene as described you would disregard the pre-exposure and give the film the exposure indicated by your first meter reading. Since you are into the realm of reciprocity you would also reduce development from what your normal time is to compensate for the increased contrast that all reciprocity exposures produce.

    Forgive the lengthy answer but I felt that it was important to convey the considerations that were involved in your question.
     
  4. dr bob

    dr bob Member

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    If the shutter speed is 1/2 sec. it should not require compensation. I have always felt that Zone II is fixed for any specific film regardless of ultimate subject illumination. Maybe I'm on the wrong track here and/or have misinterpreted the requirements listed in your example, but if I was to do this, I would photograph a gray card (or my palm?) with the camera set to produce a Zone II using whatever light is available. If the time setting is longer than it seems practical, try to get more light on the card / hand.
     
  5. Donald Miller

    Donald Miller Member

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    dr bob, I think that Annie was asking about a 1/2 second pre-exposure followed by a second exposure which her meter indicated as being 4 seconds. Perhaps I am failing to understand your answer but it seems that you are answering on the basis of only a 1/2 second exposure . Please straighten me out if I failed to understand you.
     
  6. dr bob

    dr bob Member

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    No Don, I was trying to indicate that the production of a zone II pre exposure depends only on - whatever it takes to produce it. Ex., regardless of the subject illumination, if I were about to do a night shoot, I might use a flashlight or headlights or whatever to illuminate my gray card (or whatever) then, using metered data, set the shutter and lens in the most convenient manner to get the zone II.

    I congratulate you on including the development requirements for this sort of image in your original reply.
     
  7. Donald Miller

    Donald Miller Member

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    dr bob, I understand now. Thanks for bringing me up to speed.
     
  8. Annie

    Annie Member

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    Thanks all, now I am up to speed also!