the maker claims this lens to have a 200° diagonal coverage on APS-c. (with 160° and 104° on the long and short sides). On full-frame 24×36 mm sensors, it will show a cropped circular image. Even if it’s similar, it’s NOT the same lens as the Samyang 8mm f3.5.
The same lens or something very similar is sold also under Opteka’s name, as “Opteka 6.5mm f/3.0 Ultra Wide Angle Manual Focus Aspherical Fisheye” despite the nominal values seeming different, looks and number of lens/groups are exactly the same.
With time other incarnation brands came out. Altura and Lightdow.
(thanks to Björn Crombach for the suggestion!)