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  1. #21
    ic-racer's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    I'm surprised you don't remember either thread. The 1.0 ratio is the reason why there's a consistent discrepancy between ISO speeds and Zone System EIs.

    The proof for the ratio is with exposure meter calibration and how it relates to the metered camera exposure point.
    I sometimes skip or ignore threads and posts that say "zone system" so I may have missed that. I do remember a thread where I was posting about uncertainties in zone exposure being at the beginning, middle or end of the zone, so it was probably in that thread.

    But is the "1.0 difference" a definition? If so, wouldn't all meters be required to have the same "k" constant?

  2. #22
    Stephen Benskin's Avatar
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    Quote Originally Posted by ic-racer View Post
    I sometimes skip or ignore threads and posts that say "zone system" so I may have missed that. I do remember a thread where I was posting about uncertainties in zone exposure being at the beginning, middle or end of the zone, so it was probably in that thread.

    But is the "1.0 difference" a definition? If so, wouldn't all meters be required to have the same "k" constant?
    Dale, the only way a single meter can be used for exposure with both negative and positive materials is through the speed constant. The exposure meter only wants to make one exposure at 8 lux * 1/ISO. The speed constant is necessary to determine where the exposure range falls. For black and white negative film, it's 0.8. With color transparency film, it's 10.

    There are a lot of variables that combine to make K. The K factor thread really goes into detail on this. In a nut shell, many of the factors have to do with the physicality of the meter and the camera optical system. But no matter what the value of K is, the exposure is intended to be 8 / ISO.

    Here's a page that introduces the equation for K and if you're wondering about where the value of 8 in 8 / ISO comes from, check out the value of K1.

    Click image for larger version. 

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    Last edited by Stephen Benskin; 11-01-2012 at 11:16 PM. Click to view previous post history.

  3. #23
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    I forgot to mention that the equation Eg = q*Lg / A^2 equals approximately 8. This is the same thing as Sunny 16. Eg * t (shutter speed) = Hg or Hg = 8 / ISO.

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  4. #24
    ic-racer's Avatar
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    Steve, that is great that you can understand that discrepancy (1.0 vs 1.2) and show us where it comes from. All these years I have used 1.2 and did not pickup on it in the papers I read.

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