The Canon EF 17-40mm f/4 L USM is an affordable L-series ultra-wide-angle zoom lens that's ideal for both film and digital SLRs. Superior optics are assured by the use of three Aspherical lens elements, in addition to a Super UD (Ultra-low Dispersion) glass element. Optical coatings are optimized for use with digital cameras.
This lens focuses as close as 11 inches (0.28m), and offers both Canon's full-time manual focus and a powerful ring-type USM for fast and silent AF. It has a constant f/4 maximum aperture, and offers the choice of screw-in 77mm filters or a holder in the rear of the lens for up to three gel filters. Finally, it offers weather-resistant construction similar to other high-end L-series lenses.
Ultrasonic Motor (USM)
Instead of large noisy drive trains powered by conventional motors, Canon USM lenses employ the minute electronic vibrations created by piezoelectric ceramic elements. The focusing action of the lens is fast and quiet, with virtually instantaneous stops and starts. USM lenses also draw minimal power from the camera, ensuring longer battery life.
Full-Time Manual Focus
This lens is equipped with a ring-type USM, which allows full-time manual focus in AF mode. This means you can manually fine-tune focusing as soon as autofocus is complete, without having to switch to manual focusing mode.
Wide-angle lenses and fast normal-focal-length lenses often suffer from spherical aberration. When the light rays coming through the center of the lens do not converge at the same point as light rays coming through the lens edge, the image appears blurred because there is no sharp point of focus. Canon's Aspherical elements use a varying curved surface to ensure that the entire image plane appears focused. Aspherical optics also help to correct curvilinear distortion as one might find in ultra wide-angle lenses.
Super UD Glass Element
Canon's Super UD (Ultra Low Dispersion) glass element significantly reduces chromatic abberation, which often appears in photographs as color fringing at the edges of objects.
Dust and Water-Resistant Construction
L-series EF lenses are highly dust- and water-resistant thanks to rubber seals at the switch panels, exterior seams, drop-in filter compartments and lens mounts. Moving parts, such as the focusing ring and switches, are also designed to keep out environmental contaminants, providing reliable performance under harsh conditions.
Canon lenses featuring circular aperture diaphragms employ curved blades to create a smoothly rounded opening as the lens is stopped down. As a result, out-of-focus background highlights are rendered as natural-looking rounded shapes rather than as distracting polygons. These lenses deliver smooth, consistent stop-down action (even at 10 fps), near-silent operation and excellent optical characteristics.
An inner focusing lens has the focusing lens group(s) in front of the diaphragm, allowing for compact optical systems that produce faster AF. And because the front of the lens does not rotate to focus, filter orientation remains constant. (less