Originally Posted by Nicholas Lindan
There is no reasonable way to fit VC paper to a curve - the stuff has 3 emulsions so you would need a summation of 3 emulsion curves. Most graded papers seem to be two emulsion VC-like papers - you can see the transition where the first emulsion shoulders out - so even there the situation gets complicated.

I found a power law fit works reasonably well, but the shoulder part of the curve is a bit of a problem. The physics behind the toe, 'linear' and shoulder regions are all different and if you use a function that is the same form as the underlying physics you end up with 3 functions for each emulsion.
Nicolas

I don't completely agree, because I get very good approximations with all types of photographic materials by using non-linear curve-fitting algorithms of 3rd to 5th order. The following equation was used to fit an ISO grade 4 contrast for MGIV-FB:

y=(3.365E+0+-5.329E+0*x+2.833E+0*x^2+-5.024E-1*x^3)/(3.471E+0+-8.779E+0*x+9.013E+0*x^2+-4.356E+0*x^3+8.218E-1*x^4)
R^2 = 9.999E-1

Feel free to plot this for x-values from 1.0 to 2.0 to see for yourself. As you can probably tell, I used the following equation format:

y=(a0+a1*x+a2*x^2+a3*x^3)/(b0+b1*x+b2*x^2+b3*x^3+b4*x^4)

Unfortunately, this is not helping the original question, but I'm very interested to continue this conversation via private eMails, if you like.