EF lenses are primarily designed for full frame cameras, namely the 1D and 5D series. All of Canon's best telephoto lenses are only offered as EF lenses, and are designated as L lenses (as in Luxury). The benefit to using an EF lens on a crop frame camera like the 500D is that the camera is using the sharper middle portion of the glass. There is less distortion as well. The downside is that EF lenses are bigger, heavier, and more costly.