One measure of a film's characteristic curve is how steep it is. Gamma is one name for the slope of the curve. Think of 45-degrees as 1.0 because for every unit of exposure difference on the film you get one unit of density difference on the negative. 0.0 would be if you forgot to develop the film. Infinity would be where it goes instantly from clear to black. (Actually that is unlikely to ever happen, and a special meaning is given to the term: "Gamma Infinity" - which is the "most contrast you will ever get" in that developer, no matter how long you develop it, even if it's only 1.5 or 2.0.)

Contrast Index is another term used to describe the slope of the curve. Contrast Index and Gamma just take two points on the curve and calculate the slope of them, they don't use curve fitting to describe the curve, it's just a straight-line between two points. The two differ from each other by the choice of endpoints you measure.

Contrast Index uses a couple points spread out about as far apart as the part of the film you would normally use to print from. So it has the advantage of relating development to meet printing needs. Think of a protractor with an inner arc and an outer arc that you slide left and right until both arcs touch the line at the same angle (marked as slope to the center of the arcs). That's all Rafal wants to do, but he wants to do it with a computer program instead of on paper.

Contrast Index Meter

Here is a graph of Rafal's film tests, with "C" marks on each curve showing the two endpoints used to determine Contrast Index.

The ASA triangle is also shown, it is very specific, and Rafal luckily hit it at 11 minutes. Note that even though the ASA triangle literally measures 0.62 slope by the points that define it, the Contrast Index of that same curve is 0.75 - significantly different...

If I had a Normal scene, I would develop it 7 minutes because I aim for specific CI and for Normal I would aim for 0.62 CI. If I used Gamma instead, I might arrive at a different recommend developing time for Normal, and if I were to develop to ASA standard, I'd possibly develop 11 minutes.