Definition: A lens that uses curved mirrors to reflect light back and forth and so create a long focal length within a physically compact body. The main disadvantage of the mirror lens is that the aperture is fixed and usually quite small, preventing any control of depth of field. Light intensity can be regulated to a degree by use of a neutral density (ND) filter. Otherwise, exposures must be controlled by shutter speeds. The telltale product of the mirror lens is the ringed appearance of out-of-focus highlights, known as 'doughnuts", which can be used to striking effect. Such distortion is due to a blind spot in the lens, caused by placing the front mirror in the center of the first lens element.
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