For those times that you need extreme reach of a distant subject, Kenko offers the Pro 300 3x DG teleconverter, designed to give optimal results with fixed focal length lenses of 100mm or more.
Corner resolution is stellar and deterioration of light is kept to the absolute minimum. Amazingly, with lenses of f/2.8 or larger apertures, autofocus is maintained. With all lenses having apertures smaller than f/2.8, Kenko recommends using the PRO 300 3x in manual focus mode.