The canon et 65b lens hood is designed for the e70 300mm lens f45 56 do is usm and ef70 300 f40 56 is usm 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 dedicated lens hoods are designed to match the specific focal length of the lens which they are designed for