That picture is incredible, that's the kinds of things I can't wait to see and try to get good pics of... I had never even thought about a fish eye, but you've got my curiosity going now. The only one I see at the camera shop is a 14mm Canon, or 8mm Sigma. I would imagine the 8mm would be the better purchase?
What you want is the
Sigma 10mm f/2.8 EX DC Fisheye HSM lens.
The Canon 14mm is an ultra-wide angle lens, but for the full-frame digital (1Ds or 5D) cameras or 35mm film. It's not very wide at all on a 40D, less wide than the EF-S 10-22mm which is specifically designed for the crop-body cameras. The Sigma 8mm is a full circular fisheye which is not what you want either. That's also for full-frame digital or 35mm film use.
The lens I have is here:
http://www.kenrockwell.com/nikon/105fe.htm
There's no equivalent Canon branded lens for Canon, which is why the Sigma 10mm is what you want. I have no idea how it performs, though.
It's also good for urban night time street photography. The fast f/2.8 aperture and slow shutter speeds you can hold it at make it a perfect night time lens. This one I corrected with the Image Trends Hemi plug-in to keep the vertical structures of the building straight, but left horizontal features as-is.
It makes capturing fireworks easy, LOL
(no distortion correction on this one)
And here's some more mountainous rugged terrain (no distortion correction here either)
This is 180-degrees diagonally and about 160-degrees horizontally. So a 10mm rectilinear lens with a horizontal field of view of about 100-degrees isn't going to get even remotely close to this view. You'd have to take multiple exposures and stitch. If you note closely though at the upper right, you can see some nasty chromatic abberations even at web-sized stuff. So if you're ok with 100-degrees or less, definitely stick with a rectilinear wide angle and not a fisheye.
And here's one with a full rectilinear conversion with no horizontal or vertical distortion remaining that I did with DxO software. You do lose a good bit of the wideness, but I think it'll still come out at about the equivalent of a 14mm rectilinear lens or so. That's what I've heard elsewhere. I took some test shots once to do my own measurements but have never been bored enough to go into Photoshop and see for myself. It's still way wider than 18mm either way.
They're also insanely useful in museums where you need both the fast aperture and an ultra ultra wide angle. I have a lot of examples of this too, but not online yet to share.
Fisheyes
are more difficult to work with and require more post-processing work than a rectilinear wide angle will, but they still deliver a much much wider view, and as long as you get your composition right and have the right tools at your disposal when you don't or can't, they're amazingly versatile lenses in the digital era. If you want maximum image quality and can live with a 100-degree angle of view or less, you should still stick with a traditional rectilinear wide angle lens like the EF-S 10-22 as noted above. I like even wider than that though. All of these photos are from a recent vacation to Taiwan where my wife has a lot of her family. I took 2900 photos total and we toured all over the place. My fisheye was my 2nd most used lens at around 900 photos, right behind my workhorse 18-135 lens. If only the weather had been better.
