The EF mount is probably one of the most versatile mount out there. With an adaptor you can enjoy SLR lenses from Contax, Leica, and even Nikon to name a few. I was extremely happy to find out that the Contax and Leica lenses would make a come back in the digital world. For one Contax is already discontinued and the Leica R digital back costs an arm and a leg for a somewhat dated sensor.