The Canon EF-S 17-85/3.5-5.6 IS USM is designed for use on the Canon EOS Digital Rebel, Digital Rebel XT, EOS 20D and EOS 30D Digital SLRs. It's a general purpose lens with a field of view coverage equal to that which would be given by a 27-136mm zoom on a full frame 35mm camera, so it's pretty much an analog of the well respected EF 28-135/3.5-5.6 IS USM. It covers the focal lengths most commonly used by the majority of photographers.

The EF-S 17-85/4-5.6 IS has a ring USM motor with full time manual focusing and 2nd generation Image Stabilization (IS) technology. That means it gives up to 3 extra stops of "handholdability" (the EF 28-135 IS has 1st generation IS which can provide up to 2 stops of added stability).  As shown in the construction diagram below, the EF-S 17-85/4-5.6 IS USM has 17 elements in 12 groups and it has one aspheric element with two aspheric surfaces for better control of aberrations.