when to return a lens for new copy?
I've been trying out my new 15-85mm lens for the last month or so, and I'm not as impressed with the image quality as I thought I would be. I guess I was expecting a significant improvement over the 18-135mm I've been using, but the 15-85mm seems just barely sharper (when viewing 100% crops). The zoom also seems to stick more than it should when it's pointed up (zooming in) or pointed down (zooming out) - but maybe I'm just being picky now and looking for reasons to send it back for another copy.
How do I know if my copy of this lens is a "less desirable" one? What specific lens tests would you do to tell if you have a "good enough" copy of a lens? Under what circumstances do you send a lens back hoping for a better copy? Your thoughts or recommendations would be much appreciated.