Hi, I was reading through some threads here on what to check for when getting 2nd hand lenses. And read that some people say that the same model of lens from the same brand can have different degree of sharpness depending on the copy you landed with. May I know how does one go about checking for sharpness to ensure that one has the best copy? I'm sorry if there's a similar question being asked before, maybe you could help post the thread reference here so that I can go read it up. Thanks.