Mirror lenses are long-focal-length lenses constructed using a series of lens and mirror elements. Light is reflected back and forth within the lens barrel before reaching the plane of the film or sensor, enabling the weight physical dimensions of the lens to be considerably reduced. Focal lengths of up to 2,000mm can be achieved in a relatively compact design - perhaps half the weight and size of an equivalent traditional design. The lenses are cheaper to buy than traditional telephoto lenses and are often used for astronomical photography. However the principal mirror can be quite large in diameter so many mirror lenses are short and fat.
Mirror lenses have only one fixed aperture setting, typically f/8 but perhaps f/11 or f/5.6, so exposure must be controlled by shutter speed and ISO setting. This can be limiting when photographing moving subjects, and also in poor light, where slower shutter speeds may be impractical. In some very bright environments the fixed aperture may also be a problem because a very fast shutter speed is demanded. This latter problem can however be solved by using a neutral density filter to reduce the intensity of the light entering the lens. Most mirror lenses have a slot for filters at the rear end. A further problem is that the small aperture makes the image seen in the viewfinder rather dark and consequently difficult to use for accurate focusing. Depth of field tends to be very shallow, so focusing is often critically important. Overall sharpness and contrast are also likely to compare poorly with conventional telephoto lenses.
An unusual feature of mirror lenses is that under the right circumstances they can be used to produce dramatic out-of-focus annular reflections. These are derived from the rear element which has a hole in its centre to allow light to pass through. The reflections appear as rings of light around specular highlights such as those found in sparkling water. The effect can be used creatively but is probably best not overused.