[Cellarman]

A mirror lens uses curved mirrors rather than lenses, so the light path is folded, making the lens physically much shorter. Telephotos use lenses (obviously), but are physically shorter than long-focus lenses. The problem with mirror lenses is that they usually have a fixed aperture size, meaning that depth of field cannot be changed. They also create ugly doughnut-shaped images of out of focus highlights.

They are a cheaper way to obtain a long focal length lens, but do not give the same quality as a good telephoto.