Review on Ultra Wide Angle Zoom Lens for SLR Cameras - Canon EF 17-40mm f/4L USM by Agata Koczan ᠌

To achieve the desired results with the Canon 50D, a few adjustments were necessary. It is not particularly sharp between apertures 4 and 5.6, but it is sharper than any EF-S. Only with 15-85 can it be compared, and even then, the second range serves somewhat different objectives. My aperture is almost always closed to 8-11 because the range involves use either for landscapes or indoors (of course with a flash), and in this state it is very sharp. The range involves use either for landscapes or indoors. When you need a portrait - at the long end and with the aperture wide open - then ringing sharpness is not necessarily required, and light software is even suitable for the portrait. When shooting macro with a built-in flash (if such perverts exist), there is a problem that arises: due to the size of the lens, it casts a shadow on the lower part of the frame that covers more than half of it. This can be problematic. If this may be considered a downside, then removing the cover once the hood has been screwed on is a pain in the neck =) This issue can be remedied by using a wound filter.