Some Planes
This is a lens that is seldom seen today, it seems history has passed it by. However, it once had quite a following, as it offered a long focal length (typically 500mm, sometimes 300mm or 600mm) and it was both lighter and shorter then typical prime lenses of that focal length, not to mention often cheaper.
The lens was unusual in that it was designed using a mirror (as the name ‘mirror lens’ implies). Here’s a Wikipedia entry on this type of lens:
