Zoom lenses are very convenient. The equivalent of two or more fixed focal length lenses in one unit. The maximum aperture of some zoom lenses changes as you alter the focal length, but through-the-lens (TTL) metering takes this into account to give correct exposure.

The EF 17-40mm f/4L USM is a lightweight ultra wide angle Canon EF zoom lens. It is especially well suited for use with digital EOS SLR cameras with a focal length conversion factor higher than 1.

Features
Superb optical performance throughout the zoom range
Fastest AF speed within its class (same as the EF16-35 f/2.8L USM)
Highly dust and moisture resistant
28cm minimum focusing distance at all focal lengths