Extend the reach of your telephoto lens with this APO Teleconverter 2x EX DG from Sigma for Canon EF-mount cameras and lenses. This model doubles the effective focal length while only losing 2 stops of light in the process. It also retains communication between the lens and camera, allowing users to use automatic metering and autofocus. Additionally, this teleconverter has a multi-layer coating that reduces flare and ghosting for sharp, clear imagery.
For compatibility please see this chart from Sigma which outlines which lenses can be used and the various functions possible with each.