When choosing a wide-angle lens for your DSLR, look out for the following key features.
The extreme convex curvature of the front element makes it vulnerable to knocks, especially as objects can appear further away through the viewfinder than they are. A petal-shaped lens hood adds protection.
Unlike some kit zooms, all the lenses in this group have metal mounting plates, which are more durable than plastic. Plastics are still used in the lens barrels to reduce weight and manufacturing costs.
Bigger zoom ranges are convenient, and make lenses more versatile. With wide-angle optics, however, you'll probably use their shortest focal lengths most of the time, so a large zoom range is less useful.
A low-profile UV filter protects the front element and lessens the risk of vignetting. The filter thread of most wide-angle lenses is 77mm, but the Sigma 10-20mm f/3.5 EX DC HSM takes 82mm filters and the Olympus Zuiko Digital ED 9-18mm f/4-5.6 takes 72mm.
Apart from the Olympus Zuiko Digital ED 9-18mm f/4-5.6, all the lenses tested feature a focus distance scale - really useful on wide-angle lenses in landscape photography because it enables you to set hyperfocal distance accurately.
The Canon EF-S 10-22mm f/3.5-4.5 USM, Nikon AF-S DX 10-24mm f/3.5-4.5G ED, Sigma 8-16mm f/4.5-5.6 DC HSM and Sigma 10-20mm f/3.5 EX DC HSM lenses use advanced, ring-type ultrasonic autofocus. This is fast and barely audible in operation, and gives full-time manual override.
The Olympus Zuiko Digital ED 9-18mm f/4-5.6 also enables this in AF mode.