The dedicated Vello EW-75II Lens Hood is specifically designed for use with Canon's EF 20mm f/2.8 Lens. Please click here for a complete compatibility list, and to see a list of available lens hood options.
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens, by extending and shading the end of the lens. A dedicated lens hood is designed to perfectly match the specific focal length of the lens it was designed for. Moreover, this hood features a non-glare matte finish on the interior surface, to further prevent or limit lens flare. and since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.
The "petal" shape of this lens hood maximizes the amount of hood coverage for the minimum amount of weight and hood area.