Right it's about a single point of exposure representing the entire range of exposure. A gray card is a single tone point. You need to pick a single point of exposure. What about a spot meter? It's calibrated to the same standard.
Originally Posted by Bill Burk
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? I believe that these are two seperate issues though.
I'm problably just not explaining it properly. One of the things I like about participating on APUG is that it can challenge what you know or think you know. This is what happened when I taught a class in photography. You think you understand it until you have to explain it.
How the exposure is placed on the curve is determined by P, as in P*1/ISO (of K1 in the exposure meter standard). Many people tend to misinterpret the paragraph in the exposure meter standard that reads, "This factor K1, has been determined experimentally by psychometrically selecting the "preferred exposure" for scene types, light levels, and camera and meter types covering the ranges normally encountered." 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).
Last edited by Stephen Benskin; 12-29-2011 at 10:08 AM. Click to view previous post history.