Telephoto mirror lenses have a long focal length but a short body.
The traditional view of lens design is that increased magnification requires increased length. Mirror lenses achieve good magnification but don't follow that rule, as they are short and wide.
History
A telephoto lens spreads light to form a bigger image, only a small part of which shines over the camera sensor. The more the light is spread, the greater the enlargement of the image focused onto the sensor. It is like using a projector; the image on the wall gets larger as the projector is moved further away from it. Traditional telephoto lens design is limited by the weight and length of a long lens, and it soon becomes too long and heavy to use. The mirror lens was designed to overcome these problems.
Optics
A telephoto lens needing a 600 mm gap between the front and the back of the lens would traditionally achieve this by being 600 mm long, but that makes it too heavy and awkward to use. Now, imagine the light entering the front of the lens and traveling down to the camera being bounced back to the front by a mirror, and then bounced back again to the camera sensor. The light still travels 600 mm, but it is folded into three steps of 200 mm. As a result, the 600 mm effect is achieved with a lens only 200 mm long. This is the principle behind the optical design of all telephoto mirror lens designs.
Image Quality
The design of a mirror lens means requires that it have a fixed aperture, usually around f/8. These lenses are also slower than traditional telephoto lenses, and out of focus areas of light take on an odd doughnut shape rather than flat discs.
Tags: mirror lens, telephoto lens, bounced back, camera sensor, lens design