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 see how this discrepancy (between 1.2 Zone System and 1.0 ISO) can explain 2/3 stop (of the 1 stop typical) discrepancy between ISO and Zone System speeds...

Zone System: Meter Zone V, stop down once Zone IV (0.3), twice Zone III (0.6), three times Zone II (0.9), four times Zone I (1.2)... where you expect 0.1 density speed point to exist on Zone I (four stops below metered point)...

versus

ISO: Meter, stop down 1 stop (0.3), 2 stops (0.6), 3 stops (0.9), 1/3 more stop (1.0)... where you expect 0.1 density speed point to exist at 3 1/3 stops below metered point.

Tell me... That's why I shoot TMY2 at 250 instead of 400... It's that simple?