You might still be right about the influence of flare. 18% Non-Flare gray card metering might still agree with 12% allowing for Flare in the camera. But I still believe there is no reason to tie the meter's gray to a percentage mathematically. I assert its placement was (as it should be) statistically determined. (Though I have no direct knowledge of it)
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).
Yes, but a computer screen and photographic print are both about the psychophysical middle - Munsell and CIE (CIELAB) . The scene is about the luminance range. Physical verses perceptual.
Originally Posted by RalphLambrecht
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).
Originally Posted by Stephen Benskin
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
There are four stops beneath V and three above.
It's always about flare with you people... :)
Four and 1/3 stops below metered exposure point actually. Bill, go back and re-read where I said it is about the middle in the camera image. For my money, it is a good possibility that this is where the calibration point decision originated. 12% is a stop off from the mid-point of a full range subject, but dead center of the camera image of the full range subject. I'm not saying this was the only part of the decision, especially in the early days, but I don't think it is a coincidence.
Originally Posted by Bill Burk
I think you'd have to go back before the 50's to get a full answer, and I'll bet the movie industry had
a lot to say about it. Personally, I became addicted to a spotmeter a long time ago. Can place values
wherever I want them.
Ralph! What the Monitor column represents?
I looked at the butkus site and read a few instruction manuals for early Weston meters. The manuals tell you to exclude the sky.
Now I have to concede a point.
I no longer think that averaging light meters were designed to read a statistically average scene brightness distribution.
If the arrow pointed to average scene brightness distribution like I have been saying all along -- you wouldn't have to exclude the sky. So I must be wrong.
It sounds more like the instruction manuals were written to explain how to use something in a way it wasn't originally designed.
I work for Kodak but the opinions and positions I take are not necessarily those of EKC. Especially on this thread.
p.s. It's amusing that Weston company proudly explains ASA standards for film speeds were based in part on their research, but the standards changed just enough that ASA speeds and Weston speeds don't match.
p.p.s. I would be very amused if it turned out ASA standard meter calibration point was based in part on Weston meter calibration research but changed 18% to 12%.