Now that I finally bought a densitometer, I am interested in calculating the contrast index, so I can see how my negatives compare with Kodak suggestions. (Don't worry, I will use other ways of evaluating them too, such as Adam's The Negative.) I found two formulas in the Kodak publication Monitoring and Troubleshooting KODAK Black-and-White Film Processes. More accurate: CI = 0.128 + (0.267 x D-min) - (0.969 x TD) + (0.454 x LD) + (0.183 x HD) + (0.039 x D-max) Less accurate: CI = (HD - TD) / 2.26 + 0.10 These, of course, are meant for use with Kodak process control strips, which I don't have. If I use an ordinary step pattern (21 steps) I figure D-min is the film base and D-max is any of the patches with such a strong exposure they are all the same. So which patches should I use for Toe Density, Low Density, and High Density? Or, maybe there is a different formula to use with 21 step patterns.