Matt Laur , Feb 19, 2009; 03:47 p.m.
This is going to depend on what camera (format) you're using, the focal lengths involved, the distances involved, etc. But the short message is: when you're stopped down to f/16 or f/22 on, say, an APS-C format digital camera, you're actually introducing diffraction, which robs you of some sharpness. Without anything else to go on, I'd say f/8 or f/11. But where you focus it (distance-wise), and the focal length of the lens will have a lot to do with the results, too. Well, that and good tripod technique!