Samyang / Rokinon 300mm f/6.3 mirror lens

introduction

NB. these lenses are often branded Rokinon, Opteka, Pro Optic, Vivitar, Bower, Walimex, Albinar, Falcon
this super telephoto lens introduced in 2010?? is a catadioptric mirror lens, and as with all such mirror lenses, share the following characteristics:
- manual focus
- fixed aperture (in this case f/6.3)
- 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