Canon EF 17-40mm f/4L USM Lens
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
Ultra-wide or standard zoom
The EF 17-40mm f/4L USM acts as an ultra-wide angle zoom on film or full-frame digital EOS cameras. On DSLRs equipped with APS-C sized sensors it acts as a standard zoom, giving an angle of view equivalent to a 28-70mm lens on a full-frame camera.
Close focusing
Minimum focusing distance is just 28cm throughout the entire zoom range.
Superior optical performance
The EF17-40mm f/4L USM has 12 lens elements arranged in nine groups. Super Spectra coating ensures excellent color balance and minimizes ghosting and flare. To provide superb image quality over the complete 17-40mm zoom range the lens uses three aspheric elements. Super UD (Ultra-low Dispersion) glass prevents chromatic aberrations.
Ultrasonic focusing
A ring-type ultrasonic motor drives autofocus extremely quickly, and in near silence. Excellent holding torque ensures the point of focus is reached accurately, without overshooting. Furthermore, full-time manual override is available, making it possible to adjust focus without leaving AF mode.
Non-rotating front element
The front of the lens does not rotate, which makes using specialized filters, such as polarizers and graduates, easier. A drop-in gelatin-filter holder can be found at the back of the lens.
Smooth background blur
A virtually circular aperture diaphragm gives out of focus regions a softer, more uniform feel. Sharp foreground objects stand out against a smooth blurred background.
Weather resistant
Highly resistant to moisture and dust, the EF17-40mm f/4L USM is ideally suited for use in harsh environments. Environment friendly lead-free glass is used throughout its construction.
