Features:

• High-magnification zoom lens with a 10x zoom ratio, optimized for digital SLR cameras
• Features SLD (Special Low Dispersion) glass elements for superior correction of chromatic aberration
• Features zoom lock switch to prevent "zoom creep"
• Lens construction features 20 elements in 16 groups
• Includes a zoom hood and removable tripod collar

Editor Reviews:

Product Description:
This high zoom ratio lens is optimized for digital SLR cameras and capable of covering a wide range of focal lengths from 50mm standard to 500mm ultra telephoto.

Amazon.com Product Description:
The Sigma APO 50-500mm f4-6.3 EX DG HSM Lens for Canon Digital SLR Cameras is a high-magnification zoom lens with a 10x zoom ratio. The advanced optical design of the lens is optimized for digital SLR cameras, and the super coating on each glass surface produces superb color rendition, while reducing flare and ghosting. This lens uses four pieces of Special Low Dispersion (SLD) glass for excellent correction of color aberration throughout the entire zoom range. The Hyper Sonic Motor (HSM) provides quiet high-speed AF function, as well as full-time manual focus capability. This lens also features a Zoom Lock device to eliminate "zoom creep" when the lens is tilted down. When the lens is set to the 50 millimeter focal length, it is remarkably compact and provides easy handling. The tripod mount is composed of magnesium, to minimize total weight. It is also easy to grip and carry the lens due to ergonomic design of the tripod mount. Non-rotating front lens elements, due to the rear focus mechanism, make possible the use of a custom bayonet "petal-type hood," which prevents extraneous light from degrading the image quality. This mechanism also allows the use of circular polarizing filters, since the front barrel does not rotate during focusing or zooming.

Rating : - You get what you pay for
I really wanted to like this lense. I have used it for the last 2 years in Maine - typically with less than ideal lighting. It is a great price point, but the poor low light ability and unacceptable image sharpness (compared to my Canon 70-200) convinced me to upgrade. I did get almost $900 on eBay though!

The images with the Canon 300 and 1.4/2.0 extenders are superior to the Sigma. If you want great, sharp images then you need a different lense. I just received my 5D Mark II. I will put it thru it's paces and report back.