measurable pre-flashing film technique?

Discussion in 'Exposure Discussion' started by gongman5000, Dec 20, 2010.

  1. gongman5000

    gongman5000 Member

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    I'm trying to figure out a way to reliably preflash film in camera. My basic idea is to double expose the film by shooting a grey card first. I found some EV>lux and EV>foot candle conversion tables in my sekonic light meter manual so I was thinking that if I could figure out how long i need to expose a given film at a given EV value to achieve various film densities then i could preflash the film to say 10%, 20%, 30% etc in any given lighting conditions. I've been looking at the data sheet for fuji pro 160s and am not completely sure how to interpret some of the charts, or even sure if they are exactly what I need. You guys have any ideas?
     
  2. mike c

    mike c Subscriber

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    Color film my be tricky don't want a color shift or something.With B&W I've used a Styrofoam cup over the lens or a large even lit area to expose the film on a zone 1 exposure, then reset the shutter and expose your picture.
     
  3. gongman5000

    gongman5000 Member

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    I think as long as I use an 18% grey card and shoot it under the same lighting conditions as the second exposure there shouldn't be any issues with color shift.
     
  4. gongman5000

    gongman5000 Member

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    Reading my original post it seems a bit vague. I guess what I'm trying to ask is does anyone know of any data available for specific films (ideally, fuji 160s and 400h or kodak portra nc) that documents the level of negative density achieved given an exposure time value for some photometric (lux, footcandles, etc).

    I've attached a chart from fuji on it's 160s film. Not exactly sure how to interpret the the x-axis "exposure [log H (lux-seconds)]"
     

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  5. debanddg

    debanddg Member

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  6. RalphLambrecht

    RalphLambrecht Subscriber

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    Not too difficult, but as you noticed, important to understand if one wants to interpret manufacturer's data sheets such as this one. Maybe I can help:

    Photographic exposure is the product of the illumination and the time of exposure. This can be expressed as:

    H = E * t

    where ‘H’ is the exposure required by the emulsion depending on film sensitivity, ‘E’ is the illuminance, or the light falling on the emulsion, controlled by the lens aperture, and ‘t’ is the exposure time controlled by the shutter. The SI unit for illuminance is lux (lx), and exposure is typically measured in lux-seconds (lx·s).

    The exposure shown on the horizontal axis of your sample graph is listed in log units. The log of 1 is '0', which means at x=0 there is an exposure of 1 lx·s and it creates a density of 1.5 on the red layer. A doubling (one f/stop difference) in log is 0.3 (log of 2). Consequently, you know that there is a 10 f/stop difference between '-3' and '0'. According to the current ISO standard for B&W films, finding the exposure required to create 0.1 density above base+fog is used to calculate the film speed:

    S = 0.8/H

    If this were a B&W film (color films are handled differently), we could agree that at -2.3 lx·s there is about 0.1 density above base+fog and therefore our speed point. Now perform the following calculation:

    S = 0.8/10^(-2.3)

    to get to an ISO speed of 160.

    Hope this helps.
     
    Last edited by a moderator: Dec 21, 2010