Rudi, you make a very good point, and I have no doubt that spending more time writing code that would fit a more correct curve, which follows the log relationship which you, and David, have mentioned, would be a better way to get accuracy. However, my original goal was to calculate the CI in a more automated way than what I did previously using graph paper, having picked the technique from Bill (thanks!). When doing it by hand, I already had to create a "spline" by rotating the French curve (Bezier) two or three times, as I was drawing a smooth line through my points plotted off the densitometer. Then I was using the CI-ruler to approximately find the 3 points that met the condition. I have a feeling that the Bezier spline approximation, calculated in my code, does as good, or perhaps even a better job that what I did by hand. Perhaps you have a set of logE/D points for which you know their CI, that we could use to further test my code? I wonder how much more (useful) precision would I gain by finding a better, more "film-like" log-based smoother.
Originally Posted by Rudeofus
Many thanks, David and Rudi, for your suggestions—I wonder what would be able to cope with a touch of shouldering, should it be present. I will use them the next time I have a chance to spend a day coding film-testing utilities.
Originally Posted by dpgoldenberg