Hi there, So i have seen on some occasions forum members mention that they received a 'bad copy' of a lens (say, Oly 17/1.8), sent it back and had it replaced with a 'good copy' (of the same Oly 17/1.8). How would i know if any new lens that i buy is a good or a bad copy? Are there 'test shots' i can do, and what am i looking for? As i begin to build my collection of lenses (and every so often a more expensive one) i'd like to make sure i don't get any 'duds'. Thanks!