Further to my recent mental hand-wringing about diffraction, I decided to try a little self-education. The following video shows a sequence of shots at increasing f-stop of a convenient wall in my garden (I understand walls are in fact necessary for this sort of exercise. Or cats. But they move too much. And I prefer walls).
The camera is the Olympus E-3 firmly bolted to a tripod, lens is the 12-60 set at 33mm.
Even with various levels of compression screwing around with the results, I think it is pretty clear that the image quality starts off ok, improves towards f/8, stays ok-ish until f/11, and then dramatically collapses. This is, of course, what is supposed to happen, but bearing in mind the old film-era advice of “crank it up to f/22” it is pretty scary.
No sharpening done, just standard Aperture conversion of the RAW files.