The EF 17–40mm f/4L USM lens is a wide-anglelens made by Canon Inc. The lens has an EF mount to work with the EOS line of cameras. Other than the front element, it is sealed against dust and water, and features a diaphragm which remains nearly circular from f/4 to f/8.
The 17–40mm is the least expensive of Canon's wide angle zooms, with the EF 16–35mm f/2.8L USM and the EF 20–35mm f/2.8L being both faster and more expensive. Weighing 475 g and measuring 83.5 mm x 96.8 mm, it is a popular choice with many photographers because of its light and compact size. A member of the L-series, the 17–40mm is a good substitute for the 16–35mm lens, which is heavier and costs approximately twice as much.