You are. The diffraction limit can be approximated by dividing the f stop into 1500, as it depends upon the aperture you're working at. F:2 = 750. f:4 = 375. Diffraction limit means that the lens is well enough corrected that diffraction, not aberrations, are the limiting factor for resolution. It does not mean every f:2 lens resolves 750 lp/mm
No, I think I'm not misunderstanding something, then. If the Xenar is diffraction-limited at f/8-f/11, and 1500/f is a decent rule of thumb, then that means that the lens resolves circa 150 lpmm, right? Which sounds kind of high, certainly much finer than most films could resolve.
San Diego, CA, USA
The lady of the house has to be a pretty swell sort of person to put up with the annoyance of a photographer.
-The Little Technical Library, _Developing, Printing, And Enlarging_