You probably know that stopping down (i.e. increasing your f-stop number) can increase the sharpness of your subject, but how much should you stop down to boost resolution without losing that nice, creamy bokeh? Roger Cicala did some research on this question and writes:
For those lenses that do benefit, stopping down just to f/2.0 provides the majority of resolution improvement. The difference between wide open and f/2.0 is generally much greater than the difference between f/2.0 and the maximum resolution.
Getting the edges and corners sharp requires stopping down to at least f/4 for most wide-aperture primes, and some really need f/5.6. Stopping down to f/2.8 may maximize center sharpness but often makes only a slight difference in the corners, at least on a full-frame camera.
None of the lenses performed any better after f/5.6 (for the center) or f/8 for the corners. Most were clearly getting softer at f/11.
If you’re using a wide-aperture lens, stopping down to just f/2.0 will reap big gains in sharpness while still keeping the depth-of-field narrow. Furthermore, for some lenses you don’t really even need to worry about stopping down for sharpness, since it hasn’t a relatively negligible effect on the outcome.
Stop It Down. Just A Bit. [LensRentals]