I always shoot still life so I am only discussing about sharpness and resolution at the edge here.
Lenses are usually sharper or give a better edge resolution at 1 to 2 stops below its Wide open Aperture Size at a particular focal length.
That to say, if you're using a consumer lens like EF 28-105 f4.5 - f5.6. This lens usually perform best at f8-f11.
If you're using prime lens like the EF 85 f1.8, this lens will give edge to edge sharpness all the way from f2.8- f11.
A EF 28-70 f2.8 Zoom will usually give better performance at the edge after f4.
However, there are exception to this. Certain lenses are designed to perform even at wide open. These are usually the f4 series lenses. For example, EF 70-200 f4, EF 300 f4. I think its pretty sad Canon didn't offer a 28-70 f4 or 24-70 f4 to go along with the 70-200 and 17-40.
If you will never print to S8R or larger, you can ignore all the above. :bsmilie:
i love shooting at wide open appertures for portraits....especially closesups of faces. throws the background out of focus. but with cameras with a not too fantastic flash sync speed, shooting in daylight, this might not be possible due to overexposure.
lack of sharpness wide open is not important to me... if it is, stop down a stop or two.