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1. ## A question about characteristic curve

Dear All,

I'm looking at different film's characteristic curves and I don't understand what exactly does 0 stand for on the X axis.

For example, bw400cn http://www.kodak.com/global/en/profe...4036/f4036.pdf has 0 quite on the right side, only 3 f-stops from the shoulder.

Is 0 set at the point where 18% gray at film's ISO is? Or is this point arbitrary?

Thanks

2. log (1) =0, so the zero corresponds to 1 lux-second exposure.

3. Ah, ok... so that's log exposure in lux-seconds at film plane, right? How do I translate 1 lux second to anything more familiar? Is there a rule of thumb, eg: a gray card under bright sun, f5.6, 1/100, ISO 100 is approximately 1 lux second on film plane?

4. Negative characteristics are used quite often in sensitometry. Note that in this book the unit for exposure is millilux-seconds. The former standard unit was lux-seconds. Five hundred millilux-seconds equals 0.5 lux-seconds. The log of 500 millilux-seconds is 2.7 but the log of 0.5 lux-seconds is a negative number (-1.7). All characteristic curves that used lux-seconds as the unit for exposure use negative characteristics.
Be prepared for this when looking at most characteristic curves because the change to using millilux-seconds is fairly recent and most published curves use lux-seconds as the unit for exposure. Page 17