The Canon EW-73B Lens Hood, as with other lens hoods, is made to keep undesired side lighting from entering the lens. It does this by extending and shading the end of the lens. Furthermore, since the hood extends from the lens, it gives extra protection from accidental impact. The bayonet, petal form gives maximum coverage for the lens it was designed for while adding the least amount of weight and hood area. It is removable and attachable in reverse to take up as little space as possible when storing in camera bag.Compatible with Canon EF-S 17-85mm f/4-5.6 IS USM lens.