Canon's budget L wide angle is no slouch. Prior to the existance of the 16-35mm Mark II, the 17-40L had the 16-35L beat in corner to corner testing @ f/4. The lens produces very nice edge to edge sharpness for a moderate price. Also this lens is just the right balance, not too heavy, not too light.
A new and 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.