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# Thread: Sensitometry and lens transmission formulae

1. Originally Posted by Stephen Benskin
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. Originally Posted by ic-racer
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.

3. 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.

4. 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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