The stock focusing screens (and all but a few optional ones) for AF cameras are cut with a pattern that is optimized to be nice and bright for slow lenses. That is great if you use slow lenses, but not if you use fast lenses. The screens will generally show you the D of F and viewfinder brightness at f/2.8 or higher, regardless of how fast your lens is. This means that when using lenses faster than f/2.8 in very critical focusing (up close wide open, for example), things can appear tack sharp in the viewfinder when they are not really tack sharp at the focusing plane. It happens all the time with my 50mm f/1.2 on my 10D. Try using your focus confirmation dot in the camera and see if it helps. I have learned through experience to use that dot whenever possible if I am using a lens that is faster than f/2.8.