Do let us know what model Canon body you have. Is it full frame or crop sensor? Keep in mind you can use full frame lenses (EF series) on crop sensor bodies, but not vise versa. (ie don't try to mount an EF-S lens on a 1D, 5D)
The full frame sensor cameras don't have as much backspacing as the crop sensors do, so a crop sensor lens will make contact with the mirror assembly causing damage that is not covered under warranty
The full frame sensor cameras don't have as much backspacing as the crop sensors do, so a crop sensor lens will make contact with the mirror assembly causing damage that is not covered under warranty