This hood is to be mounted to the rear of a Canon FD lens. This is to be used with a reverse ring adapter. So when you use a reverse ring adapter you're mounting the lens to the body in the opposite direction so the front of the lens is to the body and the rear is facing your subject. This allows for macro photography. The item that's for sale is the hood that would attach to the rear of the lens just like a rear lens cap. And it supplies light blocking like any other shade would. Please see pictures.