Since Canon's EF-S 17-55 mm is sizable and is supposed to project a picture on an APS-C class detector which is smaller than a 35 mm film clip, we were expecting very good test results in the vignetting category. In this case we were very disappointed to discover that for a 17 mm focal length and wide-open aperture the light falloff in corners was as high as 40% (-1.5% the stop value). Stopping down the lens helps a bit but it doesn't solve the problem. For f/4.0 the vignetting is 24%, for f/5.6 it is 17%, for f/8.0 it's still 15% and only for f/11 we reach the generally regarded unnoticeable level of 10%.
The situation is better for longer focal however it is not perfect, especially for a lens which is priced 1000$. The light falloff for a focal length of 30 mm and f/2.8 in the frame angles was 32% (-1.1 the stop value). Stopping down the lens by one stop decreases the problem to 18%, by two stops - 10%.
A very similar situation appears for the 55 mm focal length. Here, for a f/2.8 the light falloff drops to 34%, for f/4.0 - 17% and for f/5.6 it's only 7%.
