Samyang 300mm f/6.3 ED UMC CS Mirror Lens - Sample Images
Info and Reviews:
It is also sold under a number of different brands such as Walimex, Rokinon, Vivitar and Bower.
Optical design: 9 elements in 6 groups
Lens mount: Canon EF, Canon M, Nikon F, Sony E Mount, Sony A Mount, Fujifilm X, MFT (Micro Four Thirds)