Originally posted by Nimrad Seems so weird that I can cover everything around me with just two photos.
The 10mm is a diagonal fisheye, which means it sees 180 degrees from one corner to the other (but not from bottom to top or left to right). If you want to see 180 degrees all the way around, you need a circular fisheye (like the I-think-you-can't-really-get-it-for-Pentax-yet 4.5mm Sigma), which gives you a view like this:
which, as you can see, distorts the hell out of everything.
The focal length of a fisheye tells you the magnification at the center of the image; as you go away from the center the magnification decreases (so a 10mm fisheye has a wider field of view than a 10mm rectilinear lens, even though a small object at the center of the image is about the same size for both lenses).
BTW, 180 degrees isn't the limit; Nikon used to make a lens with 220 degree coverage:
Nikkor 6mm f/2.8 Fisheye Nikkor lens