The ET-138W II Lens Hood for EF 500mm f/4 Lens from Canon is compatible with the Canon 500mm f/4L EF IS II USM Lens. It extends significantly from the front end of the lens to deflect stray, unwanted light from entering the lens and creating flare in your image. It also provides the lens with a degree of protection from accidental impact and the elements. The length of the lens hood is designed specifically for the 500mm focal length of the lens.