Rafal,
I'm glad to hear that my suggestions were helpful and that you were able to implement them in R. You might want to try fitting your data to the function I suggested in the earlier thread:

D(x) = a1*ln(1+exp(a2*x + a3))
where D is density, x is exposure (on a logarithmic scale) and a1,a2, a3 are constants. ln is the natural logarithm and exp is the number e raised to the power in the parentheses.

I didn't realize it when I first suggested this function, but I later discovered that it had been introduced for sensitometry in 1920s by Robert Luther, a Professor at the University of Dresden. Over the years, it was found to work remarkably well for lots of film-developer combinations, except for the shoulder region, where it is not applicable at all. There is also a bit of theoretical basis to justify its use.

David