this super telephoto lens introduced in 1972 and redesigned in 1981 is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/8)
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
specs
800mm focal length and field of view (3.2°) at infinity focus
fixed f/8 aperture
lens elements, ? groups (multicoated)
closest focus
mm filter thread but mount turns when you focus + mm rear filter