A mirror lens is a very specialized lens that uses mirrors instead of a typical lens. Mirror or reflex lenses are available for all major brands of cameras, either because they are in the manufacturer's catalog or because adapters are available to mount a third-party lens.
Optical Design
Mirror lenses use a catadioptric design that folds the light path in order to produce a compact lens. Mirror lenses are only available in long focal lengths because they are inexpensive to build and provide much better quality than similarly priced lens-based optics. A good quality mirror lens will cost between $120 and $250, depending on the manufacturer, while a good quality 500 mm lens will start at $900.
Size
A lens needs to have a length equal to the focal length. Because it uses a folded design, the mirror-based lens is much more compact. A 500 mm or 800 mm mirror-based lens is around 5 inches long instead of 20 to 30 inches for a regular lens. When compared to high-end lenses, mirror-based optics are compact with a width of 4 inches on average.
Focal Length and Ratio
There are no mirror-based zoom lenses and the focal ratio, usually F6.3, F8 or F11, is fixed. Because of their long focal length, mirror lenses are a good choice for on-a-budget nature and wildlife photographers.
Quality
While a mirror lens cannot compete against high-end lenses, they will be above and beyond any low-end lens on the market. A lens-based optic with good optical performance is costly to produce. A 500 mm lens can have 8 to 20 different lenses in the barrel; some of those lenses need to be made of expensive low dispersion glass to avoid chromatic aberration (color fringing on the picture) and constructed to the highest precision. A catadioptric design uses a spherical mirror that can be mass produced with good quality at a very low cost. The front plate, which is designed to correct the spherical aberration of the spherical mirror (point of light would appear as fuzzy balls) is also easy to mass produce. That design is not perfect because of the central obstruction and, while good, the mirror is not perfect. Low-end manufacturers, selling new lenses for $150, cannot produce lens-based optics that match the image quality of the mirror lens despite its design flaws.
Autofocus
The Sony SAL 500F80 and its Minolta predecessor is the only lens that has autofocus. All other lenses need to be focused manually. Because of their important focal ratio, the viewfinder is usually dim and will not allow for an accurate manual focus, except in very well lit situations.
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