The Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM are both fast wide-to-short telephoto zooms designed for use on crop sensor digital cameras. On such cameras they approximate the same angle of view that you'd get from something like a 28-80mm lens on a full frame camera. This range is useful for general purpose photography from group shots to portraits and travel. It's a focal length range (and speed, at f2.8) often used by event and wedding photographers. Both the Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM are optically stabilized, which means they're quite useful in low light given their relatively fast f2.8 maximum aperture and the 2-3 stops of additional stability that the optical stabilization systems can provide.
|Canon EF-S 17-55/2.8 IS USM||Sigma 17-50/2.8 EX DC OS HSM|
|Lens Construction||19 elements in 12 groups||17 Elements in 13 Groups|
|Angle of View (diag)||78.5° - 27.9°||72.4° - 27.9°|
|# of Diaphragm Blades||7||7|
|Minimum Focusing Distance||35 cm / 13 in||28 cm / 11 in|
|Filter Size (mm)||77||77|
(Diameter x Length)
|83.5 x 110.6 mm / 3.3 x 4.4 in||83.5 x 91.8 mm / 3.3 x 3.6 in|
|Weight||645g / 22.8 oz.||565g / 19.9oz.|
|Mounts||Canon||Canon, Nikon, Sigma,
|Warranty||1 year||1 year + 3 year extension|
|Street Price (10/10)||around $1050||around $670|
As you can see from the specifications, the lenses are quite similar. The Canon lens has a slightly longer zoom range (55 vs. 50mm), is slightly longer (about 1") and slightly heavier (about 3oz).
The Sigma lens comes with a soft case and lens hood, while the Canon comes with neither (although a hood is available). Both lenses come with a standard 1 year warranty, but the Sigma also includes a 3 year extension (making 4 years in total) at no extra charge.