The EF 17-40mm f/4L USM Lens from Canon is a high quality "L" series wide angle zoom lens for full frame and APS-C size DSLR cameras. It combines one high-precision molded glass aspherical element and two 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 lens' 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 in the size of the lens. Its ultrasonic autofocus motor enables smooth and silent autofocus action and manual focus override in One Shot AF mode allows precise manual focusing without switching into MF mode.
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 that can hold three gel filters.
This lens is a Canon 'EF' series lens, indicating it is for use with full-frame cameras. However, it also works with APS-C format DSLRs and offers a 35mm focal length equivalence of 27-64mm, making it an ideal medium-aperture standard zoom lens.