The Canon EF-S 17-85mm f/4-5.6 IS USM Lens combines a very useful zoom range with image stabilization in a light and well-performing package.
Keep in mind that the 17-85 IS is an "EF-S" lens and only works on EF-S compatible Canon EOS bodies such as the **D and Digital Rebel / ***D Series. EF-S is Canon's moniker for Small Image Circle. The distance between the film (sensor) plane and the rear of the lens is shorter than in an EF lens. Many question the longevity of this standard. My expectation is that it will be around for a long time - easily long enough for most people to get their desired value from a lens.
There also seems to be confusion about how the 1.6x Field of View Crop Factor (FOVCF) relates to EF-S lens framing. The answer is that you still need to apply the "focal length multiplier" to get the conventional 35mm equivalent field of view. The focal length is based on physical design of the optics. Yes, even though the lens is designed for 1.6x FOVCF digital bodies, the framing is still equivalent to a longer focal length lens on a full frame body.
Why put Image Stabilization into a wide angle to short telephoto zoom lens? For the same reason you put it into telephoto lenses - to reduce camera shake. Since the 17-85 IS has third generation IS, it can be handheld up to 3 f-stops lower than a non-IS lens at the same focal length. Granted, a wide angle lens can be handheld at lower shutter speeds than a long telephoto lens, but being able to hold them at even lower shutter speeds is a great benefit to me. It also compensates some for the slow/narrow wide open aperture. This lens can be handheld at slow enough shutter speeds to allow significant subject motion blur when desired - such as moving water. The moving water pictures in the 17-85 IS sample gallery were all shot handheld - down to 1/3 second.
IS on this lens also allows for a very large DOF (Depth of Field) while shooting handheld. Landscape photography is one style of photography that benefits from this feature.
Pictured above from left to right are the Canon EF-S 18-55mm f/3.5-5.6 Lens, Canon EF-S 60mm f/2.8 USM Lens, Canon EF-S 10-22mm f/3.5-4.5 USM Lens, Canon EF-S 17-85mm f/4-5.6 IS USM Lens and Canon EF-S 17-55mm f/2.8 IS USM Lens. The same lenses are pictured fully extended below.
Do not plan to use the Canon EF-S 17-85mm f/4-5.6 IS USM Lens to stop action unless you have plenty of light - or plan to use a flash for the main lighting (usig the bright flash to stop the action). Also, getting a diffuse background blur is not easy with a less that only opens to f/5.6 at 85mm. It takes a close subject and plenty of open space behind the subject to get a good background blur.
The background blur quality you do get with the 17-85 IS is fine. Curved aperture blades are partly responsible for this feature. Out of focus highlights retain round shapes even when the 17-85 is stopped down.
With ring USM, the Canon EF-S 17-85mm f/4-5.6 IS USM Lens focuses very fast - and very quietly. FTM (Full Time Manual) focusing is enabled in AF mode. IS on this lens is also quiet while engaged. There is a noticeable click when IS is started and stopped, but only a quiet hum is audible while IS is functioning.
The 17-85 IS utilizes an extending but non-rotating front element. The non-rotating design feature is appreciated by Circular Polarizer Filter and split ND (Neutral Density) Filter users.
Although not a macro lens, the 17-85 IS focuses down to 1.15' (.35m) for a MM (Maximum Magnification) of .2x. Close enough to fill the lens with smaller subjects such as flowers.
The three lenses compared above are from left to right, the Canon EF-S 17-55mm f/2.8 IS USM Lens, Canon EF-S 17-85mm f/4-5.6 IS USM Lens and Canon EF-S 18-55mm f/3.5-5.6 IS Lens. All lenses are shown extended to their maximum lengths with their optional lens hoods attached. I recommend getting the Canon EW-73B lens hood for this lens. The Lowepro Lens Case 1W is a good choice for storage/transport.
Since the Canon EF-S camera bodies feature a 1.6x field of view crop factor, the 17-85 IS has the equivalent field of view/framing to a 27.2-136mm lens on a full-frame camera body - nearly equivalent to the Canon EF 28-135mm f/3.5-5.6 IS USM Lens. My observations are that this lens also performs optically similar to the 28-135. The two lenses are very similar in size - the 17-85 measures 3.6" x 3.1" (92mm x 78.5mm) (lxd) and weighs 16.1 oz (475 g). Build quality is similar as well. The two primary differences between these lenses are the focal length range and the newer image stabilization system in the 17-85.
The 17-85 IS has decent build quality. Not L class, but it has a nice quality feel.
Optical performance is also not L-class, but reasonable. Center sharpness is very good at all focal lengths and improves very little when stopped down. Some corner softness is present at most focal lengths and apertures.
Light fall-off is noticeable until stopped down or zoomed out to 35mm or so. Barrel distortion is very noticeable at 17mm and is also mostly gone by 35mm or so. CA (Chromatic Aberation) is very strong at 17mm, well controlled through the mid focal lengths and again visible at 85mm.
Flare is only moderately controlled. Vignetting is apparent on the wide end of the focal length range until stopped down.
Use the mouseover links above to compare the 17-85 IS with the Canon EF-S 18-55mm f/3.5-5.6 IS Lens, Canon EF-S 18-200mm f/3.5-5.6 IS Lens and Canon EF-S 28-135mm f/3.5-5.6 IS USM Lens mounted on a Canon EOS 50D.
The 17-85 IS is a significantly better lens than the significantly less expensive Canon EF-S 18-55mm f/3.5-5.6 Lens, and better than the Canon EF-S 18-55mm f/3.5-5.6 IS Lens, but it is not optically the equivalent of the Canon EF-S 17-55mm f/2.8 IS USM Lens or some of the L-Series lenses in similar focal length ranges. The Canon L Series Lenses will provide better sharpness (especially in the corners) and contrast at equivalent apertures in addition to having better build quality. Light fall-off is not present in these lenses at 1.6x field of view, CA is better controlled ... While not an L lens, the 17-55 is at least as good optically.
But, the 17-85 IS is going to be the right lens for a lot of people. With its wide 5x zoom range, the 17-85 makes a great walk-around lens. There is a huge range of subject matter that fits nicely in this focal length range. And in addition to being relatively small and light, the 17-85's image quality is quite good.
The Canon EF-S 17-85mm f/4-5.6 IS USM Lens is a lens I recommend for many situations.