The dedicated Pearstone LHC-ET60 Lens Hood is specifically designed for use with Canon's EF 75-300mm f/4.0-5.6 USM, II, II USM, III & III 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. A dedicated lens hood is designed to perfectly match the specific focal length of the lens it was designed for. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.
