Andreas, you might be misinterpreting Davis. Where did you see the 0.50 in BTZS?
Frequently the ZS variables are 1.20 NDR and 2.10 subject luminance range which is 1.20 / 2.10 = 0.57. While this version of the ZS average gradient is practically identical to Kodak's (and the standard model), the justification for it doesn't reflect what is actually occurring.
Here's a comparison of developmental models from Way Beyond Monochrome, the Zone System, no flare, and my Practical Flare model.
CI Development Model Comparison.jpg
Last edited by Stephen Benskin; 02-11-2013 at 08:33 PM. Click to view previous post history.