The Canon EF 17-40mm f/4 L USM is a professional grade wide angle zoom lens. With the ability to go from normal to ultra wide angle focal lengths without changing lenses it's a great tool for for architectural, landscape, & street photographers.
This lens features a plastic body, meaning its lightweight and easy to carry around for long periods of time. The optics are very good despite the fact that it's a zoom. It also features a constant aperture of f/4 which is more than enough for daylight shooting. It also features Canon's USM (Ultra Sonic Motor) drive, which focuses quickly and makes the lens nearly silent.