Sigma 1.4X EX DG APO
(From Sigma literature) These are dedicated APO teleconverters that can be mounted between select lenses and the camera body to increase the focal length by the power of 1.4x. The new multi layer coating reduces flare and ghost, which is a common problem with digital cameras. They demonstrate best performance with DG series lenses. They also offer maximum photography magnification by 1.4x without any variation in the minimum focusing distance. Compact and lightweight, these teleconverters convert your lenses into longer focal-length lenses, so you don't have to do a lot of unnecessary footwork.