Quote Originally Posted by JBrunner View Post
... Most real world sharpness limitations that I have encountered with quality optics are set by the technique, circumstance or equipment, not diffraction...
You are absolutely correct, JB.

"Diffraction limitation" is NOT linear, but much more like "falling off a cliff" when a certain ratio of aperture-to-focus lentgh is exceeded. That ratio (note that I am avoiding the idea of `f/stop`) is far greater with longer focal lengths - and that is why a 50mm lens will only have limit of f/16 and a 500mm may be stopped to f/64.

Somewhere in this mess, there is an empiracal formula for determining the location of the edge of that cliff - if I ever get my gluteus maximus sufficiently above the alligators.