Fisheye lenses

Hello All,
I have a question. I own a CANON REBEL XTI and I really want to buy a fisheye lens. But I'm not sure what to look for. Do I buy Canon brand or is there a cheaper brand name that works for Canon? And what kind should I be looking for? 8, 6, 15mm? I don't know anything about buying a fisheye lens so if anyone can help me, that'd be super!!
THANKS!
Madison
P.S. I don't really want to spend that much. So if anyone knows of some greats deals, that'd be nice.

[OsmosisStudios]

What are you looking for from the lens?

There are 2 kinds of fisheye lenses: Rectangular and circular.

Rectangular (diagonal) fisheye lenses project an image that covers the entire sensor and show 180 degrees from corner to corner.

Circular fisheye lenses show a circular image on a black background, though the entire 180 degrees is shown in that circle.

From your description of the types...I think I'm looking for a rectangular one. Thanks!

[inkista]

The focal length isn't necessarily going to be much of a guide as to whether it's a circular or diagonal fisheye. The typical lengths prior to digital were 8mm for circular and 15mm for diagonal, but now there are 8mm diagonals for crop bodies, too.

The cheapest diagonal fisheye of good quality that you can find for a crop body is probably the Samyang/Rokinon 8mm fisheye (it's also known as the Vivitar 7mm and the Opteka 6.5. Don't ask me why. They're all the same lens, manufactured by Samyang in Korea and rebadged by multiple "house" brands). You can find it on Amazon for about $260. This is, however, a manual focus lens, which means you have to focus it, you have to set the aperture manually with a ring on the lens, you can only shoot with it in M or Av modes (because the camera can't control the aperture), and your EXIF is going to have some empty fields for focal length and aperture settings.

But, as I said, $260. Most of the other new good-quality fisheye lenses are going to start around $600. This is a niche lens, and only skateboard and spherical pano shooters tend to use one heavily.

The other lenses to look at, aside from Canon's full-frame 15mm and the new 8-15 f/4L zoom (both of which are extremely expensive), would be Sigma's fisheyes. They have a DC (Sigma equivalent to EF-S) 10mm f/2.8 fisheye that's diagonal on a crop, and is (from what I've seen) sharper than the Nikon 10.5/2.8 fisheye. Sigma 8mm f/3.5 circular fisheye is designed for full-frame and is almost a diagonal on a crop-body (you do still get vignetting and black corners, but the majority of the frame is filled). There's also the Tokina 10-17 fisheye zoom lens.