Back in 1975, this whole thing was also a mystery to me. Then I subscribed to the newsletter of the inventor of Acufine and Diafine and UFG. His exposure technique was to set the ASA to 1600 and meter the darkess detail shadow(zone 3 for you zonies) and make the exposure.I then thought, isn't that the same as setting the meter for ASA 400, metering for a zone 3, and underexposing 2 stops? I came to the insight that it was all in the metering technique. Technique that was only available through the newsletter or trial and error.(posted 8094 days ago)yours recalling old memories,
Garry D. Lewis