Quick question, why does the Sigma 10-20mm f/4.0-5.6 fit on a FF body? I thought that the lens was an EF-s lens since it was for a crop body? I shot with it for fun and it was almost like a fisheye lens. Anyone have that experience as well?
The difference between a FF and APS-C camera is not only the sensor, but also some other parts are different in size, due to the different sensors. The smaller sensor in a APS-C allows the use of a smaller mirror as well. Some distances are given in a camera like sensor-shutter, shutter-mirror, mirror-lens; this gives the distance between the sensor, and the back of the lens.
Canon was the only camera maker who decided to use the space freed by the smaller mirror for the lens. This gave them more options to design the EF-S lenses, with the backdraw, that they do not work on a FF body, since the mirror would collide with back of the lens when the mirror flips up.
3rd party lens makers design their lenses to fit on all APS-C cameras, just with a different mount. Since the other manufacturers still use the sensor-lens distance from the FF cameras, these lenses still fit there. This means that you actually should be able to use every 3rd party lens made for Canon APS-C cameras and mount it on a Canon FF body.
I think especially with a wide angle lens the effect is interessting, but normaly it does not make much sense to mount a "cheap" APS-C lens on an "expensive" FF body.