I'm having some problems with visible unsharpness (i.e. no point in tack focus) on a 28-70L on a Canon Rebel XSi when viewed at 100%. Photos were at f 2.8. Things seem a bit clearer at f 3.5. The lens has UV and CP filters I picked up at best buy on it (Tiffan I think). Shots were taken at either ISO 100 or ISO 200 at 1/125. I see a couple potential causes of the problem. 1. CP filter (which was about $20) might have issues. These things use to go for $100, it's quite possible that what Best Buy is selling might be of a low enough quality to bring down the pictures. 2) The lens may naturally not be sharp when you're talking about reviewing something at 72DPI, 50 inches tall. In this case the only thing to do is tripod mount and stop down. 3) With the crop factor of 1.6 the lens at 70mm equaled about a 110mm lens. 1/125 may be to slow to handhold when large a file will be viewed at 100%. Some of the sharp images taken with this lens might have been set to 1/180. 4) Lens may need to go to Canon for repairs. I'll be doing some shooting after work to try to work out the issue. Are either #1 or #2 known issues?