Quote Originally Posted by Stephen Benskin View Post
ZS normal is around CI 0.58. Using a fixed density method for EI determination, the difference between the ISO contrast parameters and the Zone System method of determining contrast, the fixed density point will shift slightly to the left. Probably not a full Δ0.10 but contributing as Bill said. With the fractional gradient method / Delta-X criterion, there would be negligible difference in speed.

The range of EIs from most Zone System testing is generally from 1/2 to 1 stop slower than the ISO speed. I believe imprecision of using the camera's f/stop in ZS testing and experimental error are the major causes of the variance.
Right.

To clarify, a couple points:

There may not be a "real" speed difference, because fractional gradient and Delta-x say so. But because fixed density 0.1 is the Zone I target, there will be a numeric speed loss.

You lose this speed because you develop to approximately 0.58 gradient for Zone System versus approximately 0.62 gradient for ISO.

The difference between ISO and Zone System due to this fact... I agree... is "probably not as much as" Δ0.10 logE.

We can't lock down how much. One reason it can't be locked down: "Grade 2" paper of different manufacturers may have different characteristics, so the y-axis Δdensity is not locked down in Zone System standards. Sure we can give some examples of what some people get for actual difference on logE scale. But no matter what, this is going to be one of those "approximate" numbers.

Here is a new intrigue: We have a numeric calculation result in a lower EI. But the "real" influence of this factor is effectively "nothing". We changed a number we didn't have to. Later on this MUST come back to haunt us. We must have to make this up or cancel this out in practice!