Okay, here's some background first : I got the Tamron 17-50 VC a few weeks ago, and after getting the F-- error on a few occasions, I exchanged it for a new one (actually, I returned it, and ordered a new one). Today I used the new lens and noticed that when I shoot it at its widest focal length (17mm) and at f/2.8 aperture (very low indoor light, needed ISO-1600 and still got 1/20-1/40s) - focus is very soft. Initially I thought this may be a front (or back) focus issue. But I don't get it when I use flash. And I also don't get it when I zoom into a longer focal length (24mm or more). I checked out some of the pics I took with the lens I returned, but I couldn't make a direct comparison because with that lens I don't seem to have taken many at 17 mm and f/2.8 in low light. (most of those pics were in decent light). I tested with a smaller aperture (f/4) and the softness seems to improve (gets less soft). So is this normal for this lens? Or is this a back/front focus issue? I've already done one exchange, and I hate returning and exchanging stuff! Thanks for any advice.