That's fantastic Ralph, thanks. Sounds far less complicated than the
other version.
Regards
Let's say you're using a film with an advertised speed of 400 ASA and your meter is set to that speed, and expose the first frame as the meter suggests for a gray card. Then give 1/2 stop less for the next frame, 1 stop less for the frame after that, and so on. After 9 frames you can stop and fill the rest of the roll with the first exposure setting. Process the film normally.
Now evaluate the film. Now starting with the first frame, count in the following sequence 25, 35, 50, 70, 100, 140, 200, 280, 400, until you get to the frame with a Zone I density. The counting sequence will reveal your EI. Feel free to interpolate if the Zone I density is in-between frames.
Why does this work? Zone V is 4 stops away from Zone I. That means the first frame was overexposed by 4 stops in order to generate a Zone I, if the box speed is correct. Overexposing a 400 ASA film by 4 stops is equivalent to exposing it as 25 EI. With increasing frame numbers, the overexposure is reduced and you are getting closer and closer to the advertised film speed.
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