Review of the Canon 17-40mm f/4.0 L USM

www.canon.co.uk Canon is the only company to make digital cameras with 35mm sized sensors. This lens can still be used on the APS-C sensor-equipped cameras, such as the EOS 400D or 30D, but there is a shift in the angle of view – expressed as a 35mm equivalent focal length. Canon cameras produce a 1.6x magnification effect, so this lens offers the equivalent of 27-64mm. It has a maximum aperture of f/4, but it is constant throughout the zoom range. The lens includes three aspheric elements as well as Super Ultra Low Dispersion glass to minimise chromatic aberrations and fringing. Canon has also used Super Spectra coating on the elements to provide accurate colour balance. The optic has an Ultra Sonic Motor (USM) for fast and quiet AF, while the Internal Focusing (IF) means the front element won’t rotate, so circular polarising filters can be used. There’s a 77mm filter thread on the front and a filter holder on the rear for drop-in gelatin filters. Build and Handling The lens is built to a high standard. The construction is a combination of metal and plastics, with grooved rubberised grips on the zoom and focus rings. Being pro spec, the lens offers dust and moisture protection too. The focusing system using the USM is exceptionally fast, whisper quiet and accurate, with little hunting except in extreme conditions. Manual focus is quick too, with a smooth action on the focus ring, which is matched by the zoom ring. Image quality The Canon 17-40mm has one of the highest resolutions in this range and consistency throughout the zoom and aperture range. The sweet spot within this range is f/8, but fall-off from there is slow and gradual, which shows a good performance. Verdict Combine the high build quality, the USM focusing, versatility across formats and exemplary image quality, and this lens is sure to appeal to many. This is one of several similar lenses in Canon’s, er, arsenal, but by keeping the focal length within manageable limits Canon has minimised any potential problems. What Digital Camera Score 89%