The 17-85mm f/4-5.6 is usm lens doesn't have a special piece of glass inside it called "UD" which the 15-85mm f/3.5-5.6 is usm does have. The wide angle difference between the two does really mean something once you consider the crop sensor multiplication factor of 1.6X, so the 15mm end equals 24mm in full frame, 35mm film camera terms. For many people who take snap shots of friends, places, and stuff, the 17-85mm is fine for their needs and can be purchased used on EBay for $200-$250. Keep in mind the 17-85mm isn't built quite as sturdy/durable (so I've learned from reading many reviews) as the newer 15-85mm lens is. If you don't need to print 8"x10" sized pictures for framing(or larger), then you should be happy with the 17-85mm is usm. I have both lenses now and will decide which one to keep within a few months of shooting pictures. Do a Google search on both lenses, there are plenty of reviews/comparisons on the two.