Canon 17-40mm

The EW-83E Lens Hood from Canon is a 77mm petal shaped lens hood designed for the 17-40mm f/4L USM and the 10-22mm EF-S lenses.

Lens hoods are primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.