see also:

• Sony Alpha dSLR system

• this super telephoto lens is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
  • 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
• originally designed by Minolta in 1991 and re-introduced by Sony in 2006

• 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 groups / 7 elements
• closest focus 4m which gives 19cm x 28cm (7.5“ x 11.0”) field of image
• 82mm front filter thread
• rear section filter holder, drop-in type (with 42mm filter)
• built-in sun-shade / lens hood
• 89mm diameter, 118mm long and 665g (compare this with 255mm and 1.3kg for a 400mm refractor style lens)
• heavier and bigger than the Olympus OM 500mm f/8.0 mirror lens

• wikipedia
• Kurt Munger
• Luminous Landscape