I'm sure this topic has been up and around many times for a very long time... but i can't figure out how to optimize the search function to get the answer i want. I have a 5d Classic (i just learned that term 5 minutes ago from one of the ill-searched topics on POTN) and more often than not, my 24-70 back-focuses to the point that it's embarrassing to even print! I took the lens in to Canon in Irvine, but it's still having the same issue... should I take it back with the body? would that help? the puzzling thing is, when it's sharp, it's tack sharp... it does seem sharper when the subject is closer and at the telephoto side of the zoom range.... any thoughts?