Tokina 3.5 / 17mm

This is not a fisheye, but a super wide angle lens. It can cover an angle of view of 103 degrees without distortion; I do not know a lens covering 24x36 that has a wider angle of view.
Not easy to find, until now I was looking for a M42 version, the K-mount being hardly compatible with the EOS 5D; but now I have a Sony A7.

Spécifications: 11 elements in 9 groups, 305g.

The Pentax-K mount lenses are unusable on a EOS 5D with the current adapter rings, which place the iris control and its protection on the way of the mirror. Or you have to take them apart.
But this lens can be found in other mounts, more compatible. And is available in Canon AF Version!