Catadioptric lens - A type of long-focus lens that allows a very short physical length to be captured. The light path is folded by means of two mirrors, a technique used in astronomical telescopes. Highlights in an image taken with this kind of lens appear as small doughnut-like rings, because of the central hole in one of the mirrors. One of the disadvantages of mirror lenses is that the aperture is fixed. See also Mirror lens.
