If you’re a photographer using a Canon Digital Rebel camera, or one of the prosumer models like the EOS 20D to 50D, you’ll find your wide angle options limited.
This is because of the crop factor that results from using a sensor that’s smaller than 35mm film. These smaller sensors, called APS-C format, are only 22.5 x 15mm in size, whereas a frame of 35mm film is 36 x 24mm in size.
The crop factor in practise is similar to using a longer lens, or 1.6x tele converter, on your Canon digital SLR (unless it’s a full-frame camera like the EOS 1Ds or 5D series).
That’s a distinct advantage when using telephoto lenses as you’re effectively enlarging the image in your viewfinder, adding reach to your lens. So a 200mm lens on your 40D is similar to using a 320mm lens on a full-frame body.
But for wide angle shots, the opposite is true. A relatively wide lens like a 24mm, when fitted to your Rebel XSi, is now like using 38mm lens.
To cater for its customers using EOS digital SLRs with APS-C format sensors, Canon has introduced a range of EF-S lenses designed specifically for use with these cameras.
Most of these lenses, like the EF-S 18-55mm f/3.5-5.6 IS “kit” lens, are reasonably capable, but of flimsy build quality. There are no L-series lenses within the EF-S range.
Fortunately Canon has introduced two EF-S lenses that do stand out from the crowd.
The Canon EF-S 17-55mm f/2.8 IS USM is a standard zoom, with equivalent focal length of approximately 27-88mm. Its stand out feature is the constant f/2.8 maximum aperture throughout the zoom range. This, together with superior image quality, excellent image stabilization and fast auto focus, make it a favorite of portrait and wedding photographers, and any others who need to shoot in low light.

Canon EF-S 17-55mm f/2.8 IS USM zoom lens
It’s the most expensive of the EF-S lenses and is also big and heavy, but highly rated by users.
The Canon EF-S 10-22mm f/3.5-4.5 USM is remarkable because it’s a true ultra wide lens for Canon Rebel and 20D-50D users. On these cameras its equivalent focal length is 16-35mm, definitely qualifying as ultra wide at the 16mm end.

Canon EF-S 10-22mm f/3.5-4.5 USM Wide Angle Zoom Lens
This lens is also highly rated by users, with some comparing the image quality to that of a Canon L lens, while also praising its sharpness and excellent color rendition.
For those who can afford them, these two EF-S lenses certainly add a new dimension of creative possibilities for photographers using Canon Rebels, any of the EOS 20D to 50D bodies, or the new 7D.
See Canon Lens Reports for more information and user ratings.