Originally Posted by Stephen Benskin
This is 2/3 of a stop, already, towards ZS test results in lower speed than ISO test results. The film speed has not changed in this example.
2/3 stop is already gone. Just because of how you use the meter in Zone System. That is a big part of the reason.
Then, as Stephen says, flare has to be added to the mix. I also assert that for Zone System testing, you also develop for less time. This moves the 0.1 density further to the right (slower).
So while you could stop right here and see 2/3 of a stop is gone.
I believe other things move the speed difference between Zone System and ISO, left or right (faster or slower), more or less giving you a final result of Zone System having one full stop below ISO.
So once you get this one big one settled in your mind, we can start looking at the other little details. I like to finish with flare, because it's the hardest. But flare is a big one.