Lens in which some (usually two) of the elements are curved mirrors. This construction produces comparatively lightweight short fat long focus lenses. They cannot be fitted with a normal diaphragm.
A lens where the light path is bent and reflected internally to increase the focal length of the lens; a simplified system that is usually less expensive than conventional super-telephoto (300mm and up) lenses.
A type of long telephoto lens that uses several mirror optic surfaces to "fold" the light path, resulting in a very powerful telephoto lens that is relatively small.