this super telephoto lens is a catadioptric mirror lens introduced in 1980, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/8.0)
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 (but gradually changes to 320mm when closer focus at 4m - this may be an issue with cameras where you need to set a focal length for the image stabiliser to work optimally)
fixed f/8.0 aperture
3 groups, 6 elements (including mirror), S.C.C coating
closest focus 4m which gives 19cm x 28cm (7.5“ x 11.0”) field of image