Samyang 500mm 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

500mm focal length and field of view (5°) at infinity focus
fixed f/6.3 aperture
7 elements / 6 groups (multicoated)
closest focus 2m
95mm filter thread but mount turns when you focus + 34mm rear filter
119mm diameter, 98mm long and 705g
T2 camera mount
kit also includes a practical transport bag, T2 adapter for a selected system and protective caps
~$140
Samyang website

Bob Atkin's review
- the 500mm f/6.3 is a little soft, the Tamron 500mm f/8 is quite a bit sharper, but the Canon 500/4.5L is the sharpest of the three
hobography review
if outside temperature is 20F degrees below inside, you need to let the lens cool for 30 min or so before it is usable.
mounting the lens on a 36mm extension tube further improves close focus - without sacrificing infinity focus! (normally, it can focus way past infinity)
sharper, better built and with less vignetting than the Samyang 500mm f/8.0 mirror lens - although plastic base plate is dodgy