I have a few questions about
@aparat 's graph in posting #20, which shows how zones on a negative are changed to zones on a print using the paper's H-D curve.
- AA's definition of zone V is middle gray, so I would expect 18% to be in the middle of the range for zone V on this graph. But on the right side of this graph, 18% is at about the three fourths point of the zone V range instead of its middle. Why?
- Are there standard definitions of the midpoints or range-boundaries of the zones? On page 116 of Way Beyond Monochrome (2nd ed), Ralph Lambrecht provides a table of densities for the zones. I presume these are midpoints of those zones. He states that these are "standard Zone System values", implying that such a standard exists.
- On the lower-right corner of the graph, print density of 0 corresponds to Dmin. But no paper can achieve a density of 0. Is this an error?
- On the lower-right corner of the graph, zone IX is shown as extending down to density of 0. But that's impossible because the definition of zone IX is "slight density without texture", which means density is nonzero. No density (paper-white, Dmin) is the definition zone X. Is this a mistake?
Well, that's four questions.
I implemented zones in my LED-head controller, giving the user the ability to place an element on the easel on a zone. The answers to these questions will ensure I got the details perfect.
All good questions. First of all, I must apologize for the confusion. I did not mean to include that plot in #20 toward our discussion of the Kodak and Ilford ISO 3200 films. It took the discussion off course. It was just an example of how Phil Davis chose to depict the division of the "Print Zones," 2 through 8, and how it depends entirely on the contour of the selected paper curve when matched to a particular film curve. So, I guess, the short answer is to just disregard that plot

. The gradient shown is not meant to accurately convey the density of the print but instead serves as a visual reminder of how subject luminance is translated into negative density. As I mentioned earlier in the thread, it is not my favorite way of showing the relationship between negative and print.
I found the discussion of Zones and tone reproduction on p. 49 in my copy of Beyond Monochrome. The author did say that "The development of these numbers was based on a few material and equipment assumptions, and they may not be completely valid for all photographers and their material choices." In other words, while these values aren't completely arbitrary, they do not really represent a standard. I've seen this type of generalization in other sources, as well. Some photographers have a very specific sensitometric definition of the important values, e.g., "I want my Zone VIII to be 1.2 over B+F," etc., but these are all based on personal preference, rather than standard values.
I don't know exactly how Win Plotter calculates their 18% value, but, in BTZS theory, it needs to be " near the middle of the normal Zone V range." It's possible that it's simply log10(1/0.18).
Again, sorry for including that plot.