Summing up some of my feedback from Rob's "paper."
· Exposure meters use Luminance not Reflectance.
· Jack Dunn’s Exposure Manual has 9% as the average scene Reflectance from a 7 stop scene.
· Exposure isn’t intended to be placed in the middle of the B&W film curve.
· The metered exposure point is determined from psychophysical testing and not from an average Reflectance and has to do with Luminance distribution.
Just for my personal culture I would like to understand the points above, if anybody can help, with simple words, and no mathematics.
The basis where I start from, and that makes me bulk on certain affirmations (which I don't deny might be right) are as follows:
Tell me which affirmation is wrong, please.
(I only use slides);
Slides can, basically, be developed in one, correct way. There is a standard well-known way to develop a slide. That's the way you are assumed to develop it;
If you take a reflected light measure on an object, of any reflectivity, and you follow the indication of the light meter, you are going to obtain a "middle grey" image (actually, an "average grey", see below).
Be the cat white, black or grey, if you point a spot meter on it, and follow the indication, you are basically going to have a middle grey cat in your image;
That's because a light meter
must assume a certain reflectivity of the subject in order to suggest an exposure value. The light meter assumes an "average" reflectivity (be it 18% or 14% we let it apart from the moment) and gives the exposure that, on the given ISO film, gives that shade of grey.
If the cat is twice as dark as the assumption, it will reflect half the light. The light meter will make the camera expose for twice the light. The cat will be twice overexposed, will be grey just as the assumption. The same if the cat is twice as reflective: twice the reflection, half the exposure, cat just as grey as the assumption. So for the light meters all cats are grey, and all cat actually will appear grey if I follow the spot light meter on them).
To insert a little bit of mathematics:
If y is luminance, a is reflectivity, and x is illuminance, the relation should be always valid:
y = a * x.
Now, a is 80% if the cat is white, and 10% if the cat is black, and 18% if the cat is grey.
Correct exposure is the exposure that makes the grey cat grey, the black cat black, the white cat white.
The light meter cannot know the colour of the cat. It tells me: "man, I don't know the colour of the cat. I tell you the value for your ISO sensitivity, and for an middle grey cat. If the cat is white or black, you adjust. If the cat is grey and you want it dark grey, you adjust".
That said, what I really can't understand is:
Exposure is not intended to be placed at the middle of the B&W film curve. Well, exposure, for me, is that value that, in that light, would give a "middle grey" cat in my slide. With B&W you can choose different strategies (because the final print is a product, the result of two factors or more factors, film, exposure, development, paper, etc). But I do assume that with a slide the light meter gives me the "average grey placement" of the point I measure, or else I should begin from page 1 of the manual.
"Correct exposure" if the exposure that renders, on my slide, the cat of its true colour. If the cat is grey, correct exposure is the exposure suggested by the light meter.
Exposure meters use Luminance not Reflectance
Well they cannot measure anything else than luminance, I get that. But they must assume a certain, predetermined reflectance in order to give me an exposure value. They don't know and cannot know reflectance, the colour of the cat, but they must be calibrated for a given reflectance, a middle grey cat (or average grey cat, see below).
The metered exposure point is determined from psychophysical testing and not from an average Reflectance and has to do with Luminance distribution
The metered exposure is, if I get it right, determined from a certain "given by design" reflectance, which is, more or less, that "average subject reflectance" that one might expect to find in real life. If that wasn't that way, all light meters would, on average, give a mistaken exposure. On average, exposure should fit the subject. That's because the exposure is based on the average reflectivity, isn't it?
[Whether this is 18%, 14% (or even 8%) is another matter]
18% is the "psychophysical" "middle grey" perceived by the human eye. 14% might be the "average reflectivity" of the scene (a totally arbitrary "average", that's not something that can be measured really, it's only an industrial practical convention based on a certain number of tests. YMMV and your scene might have a very different average reflectance and reflectance range).
The "average reflectivity" of the world might well be a bit darker than the "psychophysical" "middle grey" perceived by the human eye.
What's wrong in these affirmations?
I ask this as a genuine way to learn.