CO wrote:
Lenses generally benefit from stopping down the aperture a couple of stops from the maximum aperture. Try taking some shots with the aperture in the f/2.8 and f/4 range. Also, check for front or back focusing issues. It's possible that your camera and lens are not well matched up and the combination is back or front focusing. Do you know exactly where the focus point(s) landed? There are plug-ins that will show the focus points.