I'm thinking it might be time for you to look into AF Microadjustment for that lens (assuming you're using a 50D/5D2/7D etc. body, which from your discussion of AF modes it seems like you are). The fact that you get sharp pics with other lenses and not this one (which is quite sharp) suggests an AF system issue to me.


Of course, I might just be suggesting AF Microadjustment because I just got a LensAlign Pro and am planning to test some lenses tonight. [:P]