I've got a Rebel XSi with the Canon 18-55 IS kit lens. I like the range of the Canon 17-85 USM IS lens though. Provided I can get a used 17-85 IS for around $250, would it be better to stick with the 18-55 IS kit lens, or go ahead and get the 17-85 USM IS lens? Another thing to note is that my 18-55 IS kit lens seems to have a problem where it adds a weird artifact in the left side when I take a landscape picture on bright days with a background like clear sky. Nothing I can't photoshop clone stamp out, but it bugs me that I have to do it everytime. (I will try to find a picture with this and post it up when I can). Any other good sharp lenses within this zoom range at a good reasonably affordable price?