Quote Originally Posted by Michael R 1974 View Post
I was thinking of a (perhaps) clearer way to explain what I meant in my post above, which is probably too wordy. I think the fractional gradient constant 0.3 produces the first excellent print under all/most development circumstances when the original scene has a normal luminance range. If on the other hand, one deliberately reduces development to manage a scene of higher than normal subject contrast, the first excellent print might only come from a fractional gradient relationship of 0.4G or 0.6G etc. In other words, the fraction, rather than being a constant, could be related somehow to the subject luminance range. Or said even another way, the fraction could be inversely related to G. This would be a possible bridge between print judgement speeds (or fractional gradient speeds) and the variable EIs people arrive at in their ZS testing using a fixed density criterion with +/- development. The speed methods deal with a standard, while applied densitometry systems like ZS deal with the management of scene luminance ranges.

Note this concerns film speeds for different Gs, not the difference between a normal ZS EI and ISO speed.

Again these are just some preliminary thoughts after reviewing the papers.
Michael, lots of questions. I'll get into them presently, but one thing I see is that you are wanting to think through the reasons why one methodology is superior to another. Unfortunately, you don't have all the information. Here are the papers involved in Jones' testing of the late 1930s and early 1940s. (And they purposely tested negatives derived from a variety of luminance ranges.)

Jones, L.A., Russell, M.E., Minimum Useful Gradient as a Criterion of Photographic Speed, JOSA, vol 25, Dec 1935.
Jones, L.A., The Evaluation of Negative Film Speeds in Terms of Print Quality, Journal of the Franklin Institute, vol 227, n 3, March 1939.
Jones, L.A., The Evaluation of Negative Film Speeds in Terms of Print Quality, conclusion, Journal of the Franklin Institute, April 1939.
Jones, L.A., Nelson, C.N., A Study of Various Sensitometric Criteria of Negative Film Speeds, JOSA, vol 30, n 3, March 1940.
Jones, L.A., Condit, H.R., The Brightness Scale of Exterior Scenes and the Computation of Correct Photographic Exposure, JOSA, vol 31, n 11, November 1941

And in addition to Simple Methods for Approximating the Fractional Gradient Speeds of Photographic Materials is what established the 1960 ASA film speed standard - Nelson's Safety Factors in Camera Exposure.

safety factors in Camera Exposure.pdf