Finding the ZS EI in Schaefer is described this way as well. Not really informing you of anything here Stephen (It's merits can be argued against some other perceived better way, but it's how I do it), but just for others who care------if, after plotting the curve, an ISO 400 film does not attain a 0.1D until Zone II, for example, then all the curve data points would then be shifted 0.3 log exposure units to the left and the curve redrawn, and an EI of 200 is then assigned. The redrawn curve then represents a characteritic as if the original curve was produced at an EI of 200 (no need to produce another curve at EI 200). The ISO 125 curve reaches 0.1D at Zone I 2/3, therefore would simply be redrawn after each data point of the curve was shifted 0.2 log exp units to the left to represent the curve at an EI of 80.
Originally Posted by Stephen Benskin
Only, the horizontal axis in Schaefer is not divided in 0.2 log exp units, as is presented by your graph, but is shown with major divisions at 0.3 log exp units (as in The Negative), the attachment shows the differences; also, there would be a Zone 0 indicated without an exposure unit applied. The log value of each exposure unit, starting at Zone I, jives nicely with zone divisions at 0.3 units of log exposure, the most important log value, of course, being log10 of 2 = 0.3. I just think it makes looking at curves easier, probably a trivial thing to mention to most, but that's just my opinion.