The Canon EF17-40mm USM is an ultra wide zoom lens designed for professional and high amateur SLR EOS users. It is especially well suited for digital SLR cameras with an imaging sensor size smaller than the 35mm format. On such cameras these focal lengths can give an angle of view approximately equivalent to that of a 28-70mm lens used on a 35mm camera, which makes it the ideal standard zoom lens. Super Spectra Coating on the elements ensures excellent colour balance and minimizes ghosting and flare. To provide superb image quality over a 17-40mm zoom range the lens uses three aspheric elements of two different types while Super Ultra-low Dispersion glass is used to prevent aberrations that can degrade the clarity. The Auto-focus works silently and quickly thanks to a fast central processing unit and you can override AF mode with manual focus should you wish. Its circular aperture can also achieve a more attractive appearance to out of focus background areas in a picture. Minimum focusing distance is just 28cm throughout the entire zoom range. It's highly resistant to moisture and dust, and environment friendly lead-free glass is used throughout its construction. Suitable for use with Canon Digital SLR cameras.