The Canon EF 17-40mm f/4L USM ($839.99 direct) is an ultra-wide to normal zoom lens that is compatible with Canon EOS SLR cameras. When used on a full-frame body, the lens provides an extremely wide 17mm field of view on the wide end with a 2.4x zoom factor that takes it to a normal 40mm field of view at the long end. It features a constant f/4 aperture and a silent focusing motor. The lens is a part of Canon's top-end L series of lenses—which are easily identified by the red band around the front element. It isn't as sharp as the Canon EF 16-35mm f/2.8L II USM, and is prone to showing color fringing around highlight edges. But if you're looking for an ultra-wide zoom and you're on a budget, some work in Lightroom can make up for its shortcomings.

The lens is short and squat at 3.8 by 3.3 inches (HD). It weighs about 1.1 pounds, and uses rather large 77mm filters. The lens can focus as close as 10.8 inches, regardless of the focal length. There's a large manual focus ring behind the front element, and a zoom ring at the base of the lens. The only control switch is a toggle to change between manual focus and autofocus. In addition to the standard front and rear caps, a lens hood and soft case are included.

I used Imatest to measure the sharpness of the lens at a few different focal lengths when paired with the Canon EOS 5D Mark III£1813.1 at Amazon UK. At 17mm f/4 the lens managed an impressive 1,962 lines per picture height, well in excess of the 1,800 lines required for a sharp photo. That score is based on a center-weighted method, and doesn't tell the whole story. The center and middle parts of the frame are excellent, but performance at the edges drops off of a cliff. At f/4 the edges are a murky, muddy 895 lines, and are darker than the center of the frame. There's noticeable chromatic aberration, shown in photos as green and purple color fringing, throughout the frame at high contrast points. Stopping down to f/5.6 improves the edges, though they are still a bit soft at 1,248 lines, improves the overall score to 2,207 lines, and reduces the fringing. It's still noticeable, but a click of the chromatic aberration reduction tool in Lightroom 5 minimizes it, and enabling the lens profile correction in that same application helps to even out the brightness of the frame and removes the 2 percent barrel distortion that is displayed at this focal length.

Zooming to 28mm improves performance all around. Distortion drops to 0.9 percent, and sharpness improves to 2,007 lines at f/4, with edges that improve to a reasonable 1,541 lines. Stopping down to f/5.6 improves things, bringing edge performance up to 1,948 lines with an overall score of 2,212 lines. There's still a bit of fringing here at f/4, but it's limited to the edges of the frame, and is minimized at f/5.6. The lens performs at its best at 40mm. It scores 2,303 lines at f/4 with 1.4 percent pincushion distortion, and edges that hit 1,784 lines. Stopping down to f/5.6 brings the overall performance to 2,470 lines and the edge performance up to 1,917 lines. The way that the lens behaves is the opposite of the EF 16-35mm f/2.8L II USM. That zoom has its best sharpness at wider angles, but suffers when zoomed in. It's more expensive, but likely a better choice for photographers who are concerned with sharpness at the edges, and who tend to prefer a wider field of view.

The Canon EF 17-40mm f/4L USM is likely to be on your radar if you are considering an ultra-wide zoom for your D-SLR, but can't stomach the price of the EF 16-35mm lens. If you're willing to live with its shortcomings, it will get the job done. When shooting in brighter light you can narrow the aperture to improve its performance, and if you use Lightroom as your Raw converter you'll find that its correction tools will help to cover up some of the optical shortcomings. Despite its higher price tag, we felt that the 16-35mm was a much better performer, with fewer compromises in its design, though it too benefits from the corrections that Lightroom has to offer. That lens lets you shoot in poorer lighting conditions, and when shot at identical apertures as the 17-40mm, images from the 16-35mm are sharper at all but the narrowest end of its focal range.