While an excellent ultra-wide zoom lens for film SLR cameras, the Canon EF 17-40mm f/4L USM Lens is especially well suited for use with EOS digital SLR cameras, which usually have an imaging sensor size smaller than the 35mm format. When used with such digital SLR cameras, the 17-40mm focal length range gives an angle of view approximately equivalent to that of a 27-64mm lens upon 35mm cameras, making it an ideal medium-aperture standard zoom lens.

The EF 17-40mm f/4L USM, like all of the Canon "L" lenses, delivers optical performance equal to similar fixed focal length lenses, with silent, high-speed focusing. Distortions and aberrations are remarkably small for a lens of such amazing capability. It combines 1 high-precision molded glass aspherical lens and 2 replica aspherical lenses to achieve the extended zoom range, while also correcting for common aberrations that arise during zooming. A Super UD (ultra-low dispersion) glass element minimizes the chromatic aberration of magnification which tends to occur with wide-angle lenses, to help eliminate color bleed along the edges of the subject and achieve high image contrast and resolution.

The 17-40's minimum focusing distance is just 11" (28cm) throughout its entire range of focal lengths, which makes it very useful for close-up work. All of the focusing and zooming action is done internally, with no changes of the size of the lens. It only uses lead-free optics to help the environment, and features a circular diaphragm for improved background blur. The housing is water and dust resistant to help ensure reliable performance for both professionals and enthusiasts. Like many "L" lenses, it uses 77mm filters, but also has a rear gelatin filter slot behind the lens mount.

An autofocus super wide angle zoom lens for both film and digital cameras

Produces 27-64mm equivalent focal length with "APS-C" size digital EOS SLR cameras (such as the EOS 20D, EOS 30D, EOS Digital Rebel, EOS 50D, etc.)

Uses 3 aspherical lens elements and Canon Super UD ultra-low dispersion glass

Provides professional levels of moisture and dust resistance

Internal zooming and focusing

Circular aperture diaphragm for better, more natural looking background blurring

Accepts 77mm filters, as well as gelatin filters upon the rear of lens