The Samyang 8mm f3.5 fish-eye was introduced in 2009. At that point Samyang was little known, mostly offering cheap lenses being resold under other names, but since then, it has built itself a reputation for making high quality, manual dSLR lenses. This also resulted now in better marketing, and more distributors.

Samyang web site Korea. German web site. UK distributor.
Since the release of the original version, a new version of the Samyang 8mm fish-eye (CSII) has been released that has a detachable hood so that it can be used as a circular fish-eye on full frame cameras.
Because of rebranding, the Samyang lenses, are sold under many other names, e.g., Bower 8mm (USA), Polar 8mm (Korea), Falcon 8mm (UK/PH), Pro-Optic (Adorama), Rokinon 8mm (US, seems to be now a major distributor of Samyang lenses in the USA), Neewer, Walimex 8mm (Germany), T.K.L./TKL (Italy), Bell and Howell 8mm (USA), Vivitar 7mm (discontinued?), or Opteka 6.5mm (the last two are the same, the different focal lengths seem to be just a marketing ploy). The Phoenix name seems to be on some of the Samyang mirror lenses.

Some specs for the 8mm fish-eye:

No electronic contacts, manual focus, manual aperture.
Frame-filling fish-eye for APS-C type cameras.
Diagonal angle of view: 180° on 1.5x crop.
On Canon (1.6x), diagonal angle of view: 167°.
Horizontal angle of view on Canon: lower estimate 140°, measured ~793 cm wide object, with 150 cm sensor-object distance. Others estimate it as 151° to 155° (or 130-135), and a vertical angle of view of about 100° - 105° (90-95).
On 4/3 cameras: 130° diagonal, 108° horizontal, 81° vertical.
Size with lens caps: Length: 100mm, diameter 80mm.
Aperture: no step between f3.5 to f5.6, from f5.6 to f22, 1/2 f-stop aperture steps.
Minimal focus distance: 30cm. Michel Thoby (link below) has a chart showing the region in-focus at different distances.

Reviews and tests.

- Review at Photozone.
- Review at Lenstip.
- Michel Thoby's tests and review.
- Jeffrey Charles' review (PDF file), has also focus adjustment information.
- Sample images with shaved and unshaved Samyang 8mm on 5D and 1D, showing the extent of vignetting. Note, as pointed out above, the new CSII version has a detachable hood.

- Forum thread with Samyang 8mm images (in German) at dslr-forum. To view some images,
a log in is required. Focus adjustment info at dslr-forum.de section. Changing minimal focus at dslr-forum.de. Due to server crash 2013, not sure if all info is still there.

FOCUS

Manual focus is not an issue, usually set it to 3 meters with f5.6 and you have a large DOF (see Thoby's info above).
A number of people have reported that the focus scale on the Samyang 8mm is off. I have found that also on my copy, with the focus ring set to infinity, its optimal sharpness was actually around 1m or so at f3.5. Since I used the lens mostly at f5.6 or f8 and set to infinity, it was actually not really an issue that I had noticed. Following the instructions of Jeffrey Charles (link above), I easily adjusted the focus by loosening the 3 screws. To monitor sharpness, I tethered the camera to a laptop and used 200% live-view to find optimal focus at 1 meter and then confirmed at 3 meters.

Samples: 100% crop (550D body) of Samyang 8mm images before and after correction, straight standard jpg from camera. Note that exposure conditions before and after correction are not the same, but the main lesson about improved focus after correction should be clear. Also, stopping down really increases DOF a lot so that precise focusing isn't really an issue anymore.
Note: after the focus adjustment on the 550D, I also tried it on my older 400D body (where I had previously used it most), and I found that it was not the same!! On the 400D the lens was better before adjustment - now I have to focus off infinity to maybe 3m to get better focus. The Samyang 8mm lens extension is so minimal (only about 0.5mm for focusing from infinity to 0.3m) that variations in the lens mount between different bodies probably have an effect! Probably some field curvature can also be noticed at f3.5.

 Additional info

Fish-eye lens design, lots of technical info on Pierre Toscani's site.