Two reasons. When the Zone System was developed, the ASA was one stop slower that it is today. The Zone System's EIs and ASA was the same. Although the reasoning was very different. When the ASA film speed changed in 1960, the ZS didn't. Now the difference in reasoning comes into play. The fractional gradient speed was lower before 1960 because of a safety factor. The elimination of the safety factor increase film speed. The Zone System has a lower EI because it uses a different exposure meter reading to film speed ratio than the ISO standard. It is larger. The difference between the metered exposure point and the ISO speed point is 1.0 log-H (3 1/3 stops). The Zone System uses 1.2 log-H or 4 stops. While the average shadow exposure is considered to fall 1.30 log-H (4 1/3) stops below the metered exposure point, flare and the exposure constant move the shadow exposure slightly to the right of 0.10 over Fb+f fixed density.
Speed Point - Standard Model.jpg Speed Point - Metered Exposure Ratio - Zone System.jpg
Last edited by Stephen Benskin; 03-09-2013 at 03:04 PM. Click to view previous post history.