As I mentioned the Rokinon (or Samyang) 12mm Fisheye in the last post, I thought I would say a few more words about this bargain of a lens, which looks like this:
As I previously said, its field of view is 180 degrees on a full frame, so what does that look like, out of the lens (BTW these are just handheld snaps that I just took to show the FOV):
The fisheye distortion can be clearly seem, but after using Fisheye-Hemi, which corrects for the verticals, you get this:
Note how well Fisheye-Hemi does in protecting the FOV.
As a further comparison, let's look at what a 12mm shot looks like from my 12-24mm WA Sigma lens: taken from the same sitting position.
Chalk and cheese.
Bottom line: if you haven't got a fisheye lens, I can certainly recommend the Rokinon option. It's fun and can produce some great looking images, especially in tight corners.