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Steve --
Purge zones from your mind if you can because they'll interfere with your learning BTZS concepts.
The negative b+f density (IDmin) is chosen to provide adequate shadow density without excessive speed loss. IDmax, representing borderline highlight "white," is found by adding the paper's Exposure Scale (ES) to IDmin. This matches the negative Density Range to the paper's ES, insuring (in theory, at least) that the negative will print comfortably without loss of detail on either end of the scale.
In Zone System practice it's customary to set IDmin at 0.1 over b+f but that tends to overexpose flat subjects and underexpose contrasty subjects. It's more efficient to relate IDmin to curve gradient so that the IDmin density decreases as gradient decreases, and vice versa. Kodak's Contrast Index method does this with a protractor-like device but it's also easy to make IDmin density a fraction (1/10th to 1/6th of the gradient value); for example (using 1/8th), the appropriate IDmin density for a curve gradient of 0.5 would be 0.06.
If you'd care to send me your email address I'll send you some more information, and I may have an extra step tablet.
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