Originally Posted by smieglitz
Get ye some standard graph paper and scale the vertical axis for density in 0.10 intervals. For the horizontal axis have a spacing interval of one stop (zone) equal to 0.30 density units. From the same graph paper make a ruler scaled identically as the vertical axis. Make an index mark at the edge of origin of the scale and mark it "A". Go to the 0.20 value and make another mark labelled "B" at the edge. At a value of 2.20 make a mark "C". (I'm relating this from memory but the last value is 2.20 IIRC. Check this in the Kodak or Ilford literature or perhaps someone else could confirm?)
I'm back at work, it is Monday (hooray ???) and I'm able to check these numbers. In the 1987 version of Kodak Professional Black-and-White Films (publication F-5) on page 19 there is a description on how to determine CI. The figures for the CI ruler I quoted last week are in fact correct.

Now, I'm trying to see where the conversation got sidetracked into one of BTZS rather than CI. As I do, I'm looking at Davis' 1981 version of BTZS and trying to see how CI fits into his system. It would appear to me that Davis is actually defining density range and subject brightness range on the basis of Ilford's G-bar rather than Kodak's CI, and that these two determinants might yield different values for DR and SBR. For example, the graphs on page 35 (under the "Curve Gradient Measurements" heading) would appear to indicate greater SBR and DR when using CI vs G-bar assuming those graphs are scaled the same. Is this correct?

If so, couldn't I just take all my CI curves and convert them to the simpler G-bar measurement and then determine SBR and DR ala Davis? Then, I'd at least be on the same page as the rest of you perhaps.

Conversely, I'm wondering if the Kodak system might actually represent the standard way of expressing SBR and DR? Or, has the Ilford/Davis method become the standard amongst photographers? I'm wondering because last year if y'all recall, I was at my wits end trying to determine what various authors meant by the term "density range" in relation to their film exposures needed for alternative process printing in various media.

Joe