Reflex lens: Light comes in though a lens element at the front, is reflected from a donut shaped mirror at the back, to a round mirror at the front, which reflects it back through the hole in the rear mirror to a lens group at the back, and then onto the film. In other words it is a reflector telescope with a camera mount.
Very short due to the folded light path. Very light due to being mostly air. A diaphragm will not work in one, so the only way to adjust light transmission is with ND filters. There is no adjustment for DOF. Filters are usually inserted at the back of the lens as the front is very large in diameter. Out of focus highlights form a characteristic donut shape that some people dislike immensely. If you don't know what one looks like go down to your local camera store or science hobby store and look at a small reflector telescope. Imagine it with out the viewing lens and mount. Though there have been some shorter ones they are usually 500mm and longer. Their popularity began with the very inexpensive Russian import MTO that Spiratone sold back in the late 60's though Zeiss had one for the Contarex long before that. Ciao, Graywolf http://pages.prodigy.net/graywolfphoto
