Yes, I think that's it. I don't think these meter or film companies are talking to each other, and their respective ideas of what a 'mid-tone' is are probably not shared by one another.
However, I will quote Ansel Adams on page 33 of 'The Negative':
"The 18 percent reflectance value is mathematically a middle gray on a geometric scale from 'black' to 'white', and it is this value that the meter is calibrated to reproduce in the final print. This 18 percent reflectance is a fixed key reference point, and functions like the 'A' of the musical scale as a universally recognized basic value.
Knowing that the meter is calibrated to reproduce this value (But as I pointed out in an earlier post, all our meters can be slightly different!), we must remember that making a reading from ANY single luminance surface in the subject and using that reading to determine exposure will cause that surface to be reproduced as a middle gray in the final print. If making we make a reading from a 'black' surface in the subject, we can expect it to reproduce not as black, but as a middle gray in the print. Similarly, a reading from a 'white' subject area will yield an exposure that reproduces that area as middle gray. The meter, again, has no way of knowing what it is reading and 'assumes' that it is an average middle value comparable to that of the 18 percent gray card."
Now, this was certainly written a number of years ago, and I don't know if the 12-13% had become an accepted figure during Adams' lifetime. But if it had, he certainly wasn't adhering to it, or so he seems to indicate.
Dear Vince,
I'm not sure that the highlighted portion has any meaning whatsoever, and even if it does, we are dealing with the psychology and physiology of vision, not with mathematics. Merely because the Blessed Ansel wrote it, doesn't mean it's true. He was a great photographer long before the Zone System, but others did all the basic research, and for my money, quite a few explained sensitometry better too.
The world's first commercially successful spot meter (SEI Photometer) didn't even
have a mid-tone index because it's a complete waste of time in a spot meter except in the rather artificial world of the Zone System. After all, no speed system is based on a mid tone. They're based on shadow values (negative) or highlight values (transparency and digital).
The only other thing that matters, after you've metered either the highlight or the shadow, is the brightness range, i.e. the other index. The Zone System was designed long after the basic reseach on the D/log E curve was done by Hurter and Driffield in the 1880s and rather after the basic research on average reflectance and the First Excellent Print was done at Kodak in the 1930s.
The Zone System is an oversimplification of sensitometry in some ways, and an overcomplication in others. The only really useful bit (which is a work of genius) is arbitrarily chopping the brightness range
of a print into Zones. The real world, of course, has no Zones, which exist only in the photographer's mind until the film is exposed and processed, and the print made.
Quite probably the 'mid-tone' for a spot meter
is an 18% grey, because 18% is an object of worship for Zonies. But as far as I recall, if you look at the instructions for using a grey card, you don't point it straight at the camera, but angle it between the subject/camera axis and the subject/principal light axis. This gives about 12%... And before the grey card, in 1940, Kodak was recommending a similar procedure with (Kodak-yellow) paper packets...
The truth, I am convinced, is that the neg-pos process is so inherently flexible that it saves the negatives of an awful lot of people who think they are being far more precise than they are.
EDIT: I've put the last para in bold because I'm sure it goes to the heart of the whole debate.
Cheers,
R.
