How is mirror lens different from normal lenses?

I'm looking at a lens that says "mirror" in the title. Now I under of course what a mirror is, but how is that used in a lens? Or is that just a term used for certain lenses, and not necessarily meaning it has a mirror in it?

Thanks!

When you look at mirror lenses, just keep looking and find a good lens. Mirror lenses are horrible.

Good explanation on this site if you want in depth details:
photocamel.com/forum/photography-talk/44...hat-mirror-lens.html

Ok I found a really good explanation of what it is here: en.wikipedia.org/wiki/Mirror_lens Pretty interesting, but I think i will take your advice and keep looking. Thanks everyone!

"Catadioptric lenses are also used for photography, where they are known as 'reflex' or 'mirror' lenses. They are much lighter, smaller, and cheaper than refractive lenses with comparably long focal lengths (> 300 mm), but at the cost of some optical compromises.
Refractive-design lenses with focal lengths above 300 mm may use as many as twenty optical elements in a housing of a length comparable to the focal length. Catadioptric designs fold the optical path, greatly reducing the size and weight of the lens, and making longer focal lengths such as 500 mm and 1000 mm more easily accessible. Moreover, chromatic aberration, a major problem with long refractive lenses, is almost completely eliminated.


An example of the Iris Blur produced by a catadioptric lens, behind an in-focus light.
Catadioptric lenses do, however, have several drawbacks. The fact that they have a central obstruction means they cannot use an adjustable diaphragm to control light transmission[11]. This means the lens's aperture value is fixed to the overall focal ratio of the optical system (typically f/8 for 500 mm designs, or f/11). Their modulation transfer function shows low contrast at low spatial frequencies. The folded optical path does reduce the length of the lens, but increases its width.
Finally, their most salient characteristic is the annular shape of defocused areas of the image, giving a doughnut-shaped 'iris blur' or bokeh, caused by the shape of the entrance pupil.
Several companies made catadioptric lenses throughout the later part of the 20th century. Nikon (under the Mirror-Nikkor and later Reflex-Nikkor names) and Canon both offered several designs, such as 500 mm 1:8 and 1000 mm 1:11. Smaller companies such as Tamron also offered their own versions. Of the major manufacturers, currently only Sony (formerly Minolta) offers a 500 mm catadioptric lens for their Alpha range of cameras. The Sony lens has the distinction of being the only reflex lens manufactured by a major brand to feature auto-focus (aside from the identical Minolta-manufactured lens that preceded Sony's production)."

Gene.Culley wrote: Ok I found a really good explanation of what it is here: en.wikipedia.org/wiki/Mirror_lens Pretty interesting, but I think i will take your advice and keep looking. Thanks everyone!

"Catadioptric lenses are also used for photography, where they are known as 'reflex' or 'mirror' lenses. They are much lighter, smaller, and cheaper than refractive lenses with comparably long focal lengths (> 300 mm), but at the cost of some optical compromises.
Refractive-design lenses with focal lengths above 300 mm may use as many as twenty optical elements in a housing of a length comparable to the focal length. Catadioptric designs fold the optical path, greatly reducing the size and weight of the lens, and making longer focal lengths such as 500 mm and 1000 mm more easily accessible. Moreover, chromatic aberration, a major problem with long refractive lenses, is almost completely eliminated.


An example of the Iris Blur produced by a catadioptric lens, behind an in-focus light.
Catadioptric lenses do, however, have several drawbacks. The fact that they have a central obstruction means they cannot use an adjustable diaphragm to control light transmission[11]. This means the lens's aperture value is fixed to the overall focal ratio of the optical system (typically f/8 for 500 mm designs, or f/11). Their modulation transfer function shows low contrast at low spatial frequencies. The folded optical path does reduce the length of the lens, but increases its width.
Finally, their most salient characteristic is the annular shape of defocused areas of the image, giving a doughnut-shaped 'iris blur' or bokeh, caused by the shape of the entrance pupil.
Several companies made catadioptric lenses throughout the later part of the 20th century. Nikon (under the Mirror-Nikkor and later Reflex-Nikkor names) and Canon both offered several designs, such as 500 mm 1:8 and 1000 mm 1:11. Smaller companies such as Tamron also offered their own versions. Of the major manufacturers, currently only Sony (formerly Minolta) offers a 500 mm catadioptric lens for their Alpha range of cameras. The Sony lens has the distinction of being the only reflex lens manufactured by a major brand to feature auto-focus (aside from the identical Minolta-manufactured lens that preceded Sony's production)."

They're pretty horrible.

Bokeh highlights is a doughnut.