The Canon EF 17-40mm f/4.0L was announced back in February 2003 as a lightweight ultra wide-angle zoom and has since become a bit of a classic lens for owners of Canon EOS digital and film SLRs.
Its ultra-wide focal range delivers spectacular coverage on film or full-frame digital SLRs, although it’s sufficiently wide to also be considered as a general-purpose zoom on APS-C bodies like the EOS 30D and 400D / XTi. When fitted to an APS-C body, it delivers a range equivalent to 27-64mm, and while the long-end is clearly shorter than general purpose EF-S zooms, it does have the advantage of being compatible with a full-frame body should you wish to upgrade in the future. Read the whole review here