this super telephoto lens introduced in ? is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/4)
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
500mm focal length and field of view (5°) at infinity focus
fixed f/4 aperture
lens elements, ? groups
closest focus 15m (3m via an extension tube)
rear filter thread
T lens mount thread
159mm diameter, 216mm long and 2.7kg (6lb)
comes with a special lens cap that has a removable inner section. Leaving the lens cap in place, but removing the inner section, cuts the mirror's aperture to f/5.6