Quote Originally Posted by Stephen Benskin View Post
What I believe you might be thinking about is the bell curve of scene luminance range distribution. That it also tends to equal the exposure mid-point. Would that be the mean equals the average? Happy coincidence? What about unbalanced scenes like snow scenes?

How the exposure is placed on the curve...Sometimes it is referred to as being an "arbitrary" value, but it's a scientifically arbitrary value. And the psychometrically selected "preferred exposure"? Think Jones and the first excellent print test (and subsequent such tests).
Yes it is all about the bell curve of scene luminance range distribution. Not the actual for an individual shot today, but the average considered when the first light meters were made. Unbalanced scenes are not covered. They end up in bad exposure (unless you use a gray card or incident meter).

It's NOT in the middle. It is a full stop up from the middle. But wherever it is - it is there for coincidental, arbitrary or scientifically chosen arbitrary reasons.

The scale on the Weston Master III progresses

Which corresponds to

Zone System

There are four stops beneath V and three above.