Samyang 800mm f/8 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/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

800mm focal length and field of view (3°) at infinity focus
fixed f/8.0 aperture
8 elements / 8 groups (multicoated)
closest focus 3.5m
105mm filter thread but mount turns when you focus + 30.5mm rear filter
111mm diameter, 145mm long and 946g
T2 camera mount
kit comes with a practical transport bag, T2 adapter for the selected system and protective caps.
~$190
Samyang website

hobography review
dpreview forum post
- “reputation for being impossible to focus, and blurry to the point that there is no micro contrast”
- post shows how he made a geared focus microtuner and a rotating (off-set) aperture to address some of these issues
if outside temperature is 20F degrees below inside, you need to let the lens cool for 30 min or so before it is usable.