Optical Spatial Frequency Filtering of Image Sensors ?

Ok, I am a bit out of my depth here and I am sure you understand the detail better than I do. My point was that you can indeed model it as a gaussian and that will get you in the ballpark - but that's nothing new and what everybody does: pretend it's gaussian and convolve/deconvolve.

With Frans' model, on the other hand, you are actually closer to the real PSF and can therefore organize your information that much better. No?  It may in the end very well be that the gaussian components of the MTF overwhelm the more specific ones, but it did not look that way in the post, especially for the D40.