[Ian Grant]

It would surprise me if the ISO standard speed was the same as the EI used by ZS practitioners.

Because I think that the vast majority of photographs are taken by people who would most likely choose prints that are different then the prints preferred by most ZS practitioners.

Most people prefer lighter prints with more contrast. People who like "fine" prints tend to have different tastes.

The ISO standard allows the film speed to be test by either the ASA/BS system (which Kodak had relaxed) or the more practical DIN system used by Agfa.

So with Agfa films like APX100 where the speed was tested using the DIN standard my Zone system tests gave me an EI thge same as the ISO speed, and many others found the same. The opposite was true with Tmax100 where I needed to use50 EI instead of the box 100 ISO.

It's also worth remembering that the developer used for the ASA tests gave slightly better film speed and shadow detail than D76.

Ian