The biggest difference between the Zone System and the standards that give us ASA speeds and time and temperature recommendations...
Is the difference between specifics and averages.
The Zone System is based on metering specific light values that actually exist in a specific photograph and developing as needed for that specific photograph.
While the standards system is based on statistical study of average subjects and developing as needed to satisfy viewers of the best prints that can be made from the average negatives.
The different systems came about approximately the same time, but from ideas of different foundations.
In fact, Leigh is right about there not being 7 Zones. The fact that common practice may deal with 7 Zones is an attempt to reconcile the two different systems. Minor White called N development as the baseline which includes Zones 0 and VIII which you might call 8 or 9 steps or zones depending how you count. The fact it worked is that Minor White had you work from pitch black to pure white on the print.
The idea of working with Zones II to VIII came later and probably part of reconciliation to make Zone System more practical as it evolved.
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