The EF 17-40mm f/4L USM lens is a wide-angle lens 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 cheaper of Canon's wide angle zoom range, with the EF 16-35mm f/2.8L USM and the EF 20-35mm f/2.8L being both faster and more expensive.
While a member of the L...
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Canon EF 17-40mm lens
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