Olympus M. Zuiko Digital 1.4x Teleconverter MC-14
The MC-14 (1.4) teleconverter is used most often on the Olympus M. Zuiko ED 300mm f/4.0 IS PRO lens to make it a 420mm lens (840mm at 35mm equivalent). I find my autofocus speed, image quality, and sharpness are all outstanding with this combination.