The Canon 17-40L lens is a wide angle zoom lens with a fixed f/4 maximum aperture. In the context of L lenses, which are typically very expensive, this is one of the most affordable and retails for about $700 USD. I purchased this lens to have an ultra-wide angle travel zoom and have shot with it for over a year.

Physically, the lens is medium sized and weighs about 1 lb (Figure 1). The barrel is approximately 3.8″ x 3.3″ inches in length and diameter. With the hood, the lens is quite large and impressive looking, especially compared to consumer zooms.

Note that my comments in this review refer to the performance on full-frame cameras that have sensors with the equivalent size to 35mm film frames (36mm x 24mm). Many Canon cameras use APS-C sized sensors (22.5mm x 15.0mm) which change the angle of view considerable and effectively crop the edges of the image. All the photos in this review were taken on a Canon 5D which is a full frame camera.

Figure 1. The Canon 5D camera with the 17-40L lens mounted.

Sharpness

To test the lens’ sharpness, I selected the north entrance of Stanford University’s Quad (Figure 2) and took multiple pictures at varying focal lengths and f-stops. As I increased the focal length, I moved backwards to keep roughly the same composition in the image. I used a Canon 5D, tripod, remote release, and bubble level.

Figure 3 shows pixel level detail for a crop near the center of the image. Examining these crops, we can see that the image is sharp wide open at f/4. Stopping down improves detail but the image is already quite good at f/4 and there is little to improve.

Figure 4 shows the detail for a crop on the middle right of the image. The image shows some softness at f/4 but improves as we stop down. The middle of the range at 24mm is clearly better than the image at either 17mm or 40mm.

Moving towards the edge of the image, the quality quickly degrades as shown in Figure 5. In this case, the image is basically mush at f/4 although stopping down to f/8 helps a little. As with Figure 4, the lens appears to be best at 24mm and performs poorer at either extreme.

In general the lens performs very well from the center up to the point marked by the middle crop in Figure 2. The edges however become soft and can require reducing the aperture several stops to resolve detail.

Figure 2. The north entrance of the Quad at Stanford University taken at f/8 and 200 ISO. Red squares indicate 100% crops shown in Figure 3, 4, and 5.

f/4 f/5.6 f/8 17mm 24mm 40mm

Figure 3. Pixel level detail from center crops shown at 100% magnification.

f/4 f/5.6 f/8 17mm 24mm 40mm

Figure 4. Pixel level detail from crops near the middle right of the image shown at 100% magnification.

f/4 f/5.6 f/8 17mm 24mm 40mm

Figure 5. Pixel level detail from the edge shown at 100% magnification.

Vignetting

Figure 6a shows vignetting of the 17-40L lens when taking a picture of the sky at 17mm and f/4. This is pretty much a worst-case scenario and darkening of the corners is very pronounced. Although the vignetting is quite strong, it’s not something that I generally worry about as the plain sky tends to exagerate the differences. Furthermore, vignetting is easy to correct to in software and most RAW processors have the capability. Figure 6b shows the corrected image (ACR 4.5 with vignette correction set to level 70). Note that at 17mm the lens has a field of view of 104 degrees and it’s very difficult to find a stretch of the sky with completely uniform illumination so even with correction I would not expect the image to have exactly the same brightness across the frame.

Figure 6a. Vignetting at 17mm and f/4.

Figure 6b. Vignetting at 17mm and f/4 corrected with ACR 4.5 (amount=70).

Distortion

Figure 8 shows the distortion in the 17-40L at varying focal lengths. Each image is a full-width crop of the top portion of the picture (about the top half). As we zoom from 17 to 40mm, the lens exhibits moustache distortion at 17mm but becomes almost neutral by 24mm. At 40mm there is mild pincushion distortion.

17mm   24mm   40mm

Figure 7. The top portion of images taken at 17mm, 24mm, and 40mm showing the distortion present in the 24-105L.

Sample Images

I love using wide angle lenses in my own work and Figure 9 shows a few images taken with this lens. Click on the pictures to see a larger version.

Light beam inside a kiva at Mesa Verde, Colorado, 17mm, 1/25 sec, f/4, ISO 800	Bristlecone pine at Bryce Canyon, Utah, 22mm, 1/200 sec, f/16,
San Jose City Hall, 17mm, 1/200 sec, f/14, ISO 200
Figure 8. Example images with the Canon EF 17-40 f/4L lens. Click on the photos to see a larger version.

Other Comments

• The lens hood is huge making it difficult to store on the lens in the reversed position. However the petals do not seem very deep compared to the area they are covering so I wonder about the hood’s effectiveness.
• Combined with the Canon 24-105L, this lens makes a nice travel kit. However, I’ve noticed significant color differences between these two lenses even when everything is set on auto white balance.

Conclusions

I would rate this lens as average. In the center, the performance is excellent and it yields a very sharp picture. However, the poor edge quality is bothersome to me: I often compose with the main subject near the frame of the picture although I ususally don’t put it right at the edge where performance is worst. Unfortunately, with this zoom range there are not many alternatives except for Canon’s 16-35mm f/2.8 lens which is twice as expensive.