Diffraction

An odd and irritating fact about photographic sharpness is that even the world's best lens won't get around "diffraction" unsharpness, which means that pictures get less sharp at small apertures.
I remember in my youth wanting to make the sharpest pictures I could, and I put the camera on a tripod and stopped down to eleven, and I surprised and frustrated when they just weren't very sharp.
For most lenses, due to compromises in design, a middle aperture like F:5.6 or F:8 is a good bet for sharpest pictures. But the very best ones can actually be sharpest fully open, due to diffraction.

I asked clever-head Ctein (k-tein) about it, and he's writing a series of articles about it, here's the first one. I have to admit it some of it goes above my head.
If you're interested, don't miss the comments under the article.