Quote Originally Posted by Michael R 1974 View Post
As you know based on my many previous posts on the subject, I think Jones had it right when it comes to the underlying concept. Local contrast is what matters, not a density. Which is what led me to start this thread. How that principle evolved into practice though, I'm still not sure I "agree", which may ultimately help to bridge the gap between ISO and ZS-style personal EIs.
Jones was the first ASA chairman of the Subcommittee on Photography. Not to start any conspiracies, but he probably exerted a great deal of influence during his tenure on which methods were adopted. But the answer to most questions about the adequacy of the speed point, the speed point is a place to calculate the speed. It doesn't necessarily imply where the shadow exposure is supposed will fall. Fractional gradient method defines the mininum useful gradient. The fractional gradient film speeds are 1/2 of what they would be using the current ISO method. What many of the popular methods tend to miss is the speed constant which actually determines where the exposure is supposed to fall. (more on this later)