I'm looking at fisheye lens for a Nikon D5100. I found a Rokinon 8mm Fisheye Lens?

I'm not sure if Rokinon is a good brand. I want a fisheye lens that does the full 180 degrees. I'm not sure if all fisheye lens do the full 180 degrees, I'm also not sure if 8mm is good. I'm not sure how the numbers work, I do not know if the higher the number the better or the lower the umber the better.

A fisheye by design covers a full 180 degrees, but when using a cropped sensor camera like most affordable dSLR, it covers the 180 degrees on a diagonal, not the full circle 180 degrees that you can capture using a full frame dSLR

That said, a good fisheye can cost from $800 to $1, 200

The Rokinon costs substantially less than that. While it is NOT the best fisheye, it can run circles around the cheap ($40) add-on lenses sold as fisheye lenses but are not.

Its made by Samyang in Korea. They market it under several names, Rokinon being one of them. They are an acquired taste being manual lenses. I have the Samyang 14f2.8. I only bought it to see if all the reviews about IQ were correct. They are. I also have the Canon 15f2.8 fisheye and the Canon 16-35f2.8L II. I love all 3. I can't find a Nikon review for you but below is the review for one on a Canon crop. I think you'll be pleasantly surprised by this lens. At $420 my 14f2.8 is the cheapest lens in my collection but there's nothing cheap about the photos it produces.
http://www.photozone.de/...ang8f35eos