Extend the reach of your telephoto lens with this APO Teleconverter 1.4x EX DG from Sigma for Sigma SA-mount cameras and lenses. This model will increase the focal length by about 40% while only losing 1 stop 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.