EF 17-40mm f/4 L USM
Ultra-wide zoom lens.
This ultra-wide lens lets you capture great expanses of space, whether in the wild or in the city, and is useful for cramped interior shots, too. The aperture is a constant f/4 throughout the zoom range, and creates pleasant background blur. For optimum image quality, the lens uses UD and aspherical elements, which correct distortion and reduce chromatic aberration. For focusing, the Ultrasonic Motor ensures quick and silent operation.