A couple of comments and responses.
- AFAIK, there is a hard lower limit of f/0.5 ... but in practice, I doubt we will ever see below the f/0.95 of a few exotic lenses available for M-mount, micro Four Thirds etc.
- My apologies for introducing the simplistic idea of a single "sweet spot" aperture, based on the idea of optimizing resolution (MTF?) for an exactly on-axis, in-focus subject. In other words, mostly or entirely a balancing of diffraction vs spherical aberration. I agree with the idea that the "f/11 MF lens sweet spot" idea is probably a threshold for no visible aberrations, including coma and astigmatism in the corners and edges.
- As I have noted before, simply scaling down a lens design reduces aberrations at equal f-stop. On the other hand, aberrations at equal effective aperture diameter (giving about equal diffraction and OOF effects) are almost certainly lower for the larger format version of the same lens design. For one thing, I believe that chromatic aberration is equal at equal f-stop, but I wonder if one of our telescope/lens optics students can comment on that.
- For us in the 99% (of camera buyers, ranked by budget constraints), the relevant terms of comparison for high quality lenses is sometimes to look at alternatives of roughly equal cost. Here, a smaller format can afford to use materials of higher price per unit weight, say by having more lens elements that use exotic glasses, or more elaborate fabrication procedures. I believe that some aspherical lens elements are molded, requiring cooling in the mold, and that would probably by distinctly easier and cheaper to do in smaller sizes, due to factors like the way cooling time and uniformity scales with size. For example, Olympus seems to be pushing the use of ever more elaborate aspherical designs in m43 lately.
- For extreme close-ups ("macro") there is a fairly clear win for forming and recording a smaller, higher resolution image and then enlarging more later, since in this realm the small apertures needed for adequate DOF control aberrations well. This is a major reason for my preference for smaller pixels and optimizing lens resolution in l/mm rather than l/ph (not necessarily meaning using a smaller sensor though: I can crop.)