The EF-S series of lenses, by Canon, are exceptional lenses applicable for Canon digital SLR cameras such as: the EOS 20D and Rebel models. Why? The key design feature aims to match the APS-C imaging sensor with the image circle. How? This matching objective is achieved by reducing distance between the rear of the lens and the focal plane, thus decreasing the size of the image circle.
The result? Canon's EF-S lenses are more efficient in terms of: weight and cost of wide-angle lenses while ensuring superior image quality, resolution and contrast at all focal lengths.
As a member of the EF-S family of lenses, the 17-85 F4 IS USM is a strong performing wide-angle to telephoto lens. Its list of features include: Image Stabilization for hand-held situations; and is optically optimized for the digital SLR camera; a ring-type USM designed for fast and silent auto focus. The full-time manual focus in AF mode is also a useful addition.
The EF-S 17-85mm f/4-5.6 IS USM lens provides a 5x zoom ratio and an angle of view equivalent to a 27-136mm lens on a 35mm camera. Also present in this lens is a glass-molded aspherical lens element with aspherical surfaces on both sides. The result, superb image quality through the full zoom range. The focusing distance is impressive at a distance of 12 inches at all focal lengths. Finally, the EF-S 17-85mm IS lens, when compared to the EF 28-135mm IS lens, is 5% shorter and 12% lighter, and accepts 67mm filters.
Canon SLR Lens Overview - EF-S 17mm - 85mm F-4.0-5.6 USM IS Image Stabilized Autofocus Zoom Lens