I've read a lot about "bad copy" and various articles, and I know people generally don't like reading questions about it, but I've just received a 2.8 zoom lens, and the lens at 2.8 is so bad I cannot use it. I tested it on various lighting condition, and found that at f/2.8 it's so soft and blurry its really unusable under almost all conditions. However, as soon as I stop it down to f/4, it is very sharp. I've compared my image to the sample image on the internet, and found that my lens is more blurry (both at f/2.8). But at f/4, it's excellent, no problem at all. I'm perplexed because I don't know if I should keep it. If I do, I won't be using it at f/2.8. What constitute a "bad copy"? If it's sharp at some stopped-down apertures, is it by definition not a bad copy? If it is "bad", is it bad at all aperture? Or is it possible for some defects to affect only at the widest aperture? Thanks.