see also:

• super telephoto mirror lenses

• 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/13.5)
  • 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

• 2000mm focal length and field of view (1.25°) at infinity focus
• fixed f/13.5 aperture
• 6 lens elements, 4 groups
• closest focus 20m (0.12x macro)
• 52mm rear filter thread
• built-in ND filters (-1 stop, -1.5 stop, -2 stops)
• built-in skylight, yellow and red filters
• Pentax K lens mount
• 180mm diameter, 520mm long and 8kg
• NB. the later Pentax-M version is virtually identical