this super telephoto lens introduced in mid-1970's is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/11)
virtual elimination of chromatic aberration as uses mirror rather than lenses
also means focus position for infrared photography is the same as for light photography
central mirror:
allows much more compact lens length as the optic path is reflected internally twice
by necessity, blocks some of the light and thus causes:
decrease in image contrast
characteristic “doughnut” shaped bokeh imagery in out of focus areas
designed by OPCON Associates, a company founded by Ellis Betensky, who used to be a senior optical designer at Perkin-Elmer
their ruggedness is partially due to their solid, one-piece construction, unlike most production mirror lenses
specs
800mm focal length and field of view (3.1°) at infinity focus
fixed f/11 aperture
lens elements, ? groups, VMC coating
closest focus 7m
lens cap ?112mm
rear filter
needs to always be used with a rear filter attached, because the optical design relies on the presence of one more glass element with specific thickness
T lens mount thread
may not be able to mount on some cameras if the built-in flash of the camera protrudes too far