this super telephoto lens introduced in 1982 is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
manual focus
fixed aperture (in this case f/8.0)
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
the Olympus OM model is well recognised as being optically among the best made for 35mm cameras
it is substantially smaller and lighter than the Canon FD Reflex 500mm f/8.0 mirror lens and unlike the FD lens, it can be used on Canon EOS full frame cameras and can use conventional filters
in 1982 it was priced at $A620 which was more expensive than the OM 300mm f/4.5, just a touch cheaper than the 35mm shift lens and half the price of the OM 400mm f/6.3 lens.
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
5 elements in 2 groups (multicoated)
closest focus 4m which gives 19cm x 28cm (7.5“ x 11.0”) field of image
72mm filter thread but mount turns when you focus
built-in sun-shade / lens hood
81mm diameter, 97mm long and 590g (compare this with 255mm and 1.3kg for a 400mm refractor style lens)
if using on an OM SLR, the f/8 dim image is best managed by using a Type 1-4 focusing screen