Originally Posted by Bill Burk
Thanks for the illustrations with the different indices points' we've been talking about shown in overlays on the graphs! Thanks Chuck for providing the excellent example.

If you illustrated this with just a D-76 graph, the wrong conclusion might be reached. With only D-76 you would think there is no difference which method you use. But HC-110 shows clearly that when the curve is not a straight line, you can get different results almost 0.10 different.

Now I have no illusions about my own process. I have 0.10 variation in my process when I am "out of control" but even in that situation my negatives are "fine".
I'm glad that I was able to contribute positively, in some way, to this thread, even though I didn't realize it at the time.

Regarding the CI overlay that Stephen used on my curves, that's neet. I have calculated CI in a way that I learned from Kodak publication F-5, "Kodak Professional Black and White Films" page 17, if anyone has it. Sounds complicated when verbalized, but, quote:

"draw a straight line parallel to the horizontal axis at the base-plus-fog level...........Then mark a straightedge---the edge of a piece of paper works well. Put the following marks (using the same scale as that on your graph paper) on the straight edge---a mark at zero, one at 0.2 density (log exposure) unit, and one at 2.2 density units. Place the straightedge on the characteristic curve so that the 0.2 mark is on the toe and the 2.2 mark is on the straight-line portion. Slide the straightedge up and down, keeping the marks on the curve, until the zero mark falls on the base-plus-fog line. Mark these three points and drawa straight line to connect them. The slope of this straight line is the contrast index."

Then, of course, determine the slope of the line using rise over run.

Although I don't use CI for any decision-making I fine it interesting to know. How would a person get one of those contrast index gauges?