A professional grade teleconverter designed for ideal use with fixed focal length ultra-telephoto lenses. Will not work autofocus with these Canon lenses: 50 f/2.8 EF macro, 100 f/2.8 EF macro(non-USM), 300mm f/4.0 IS USM & 85mm f/1.8 USM. Will not autofocus with any lens slower than f/4, and will only AF with an f/4 lens if used with EOS 3, 7/7e or 1V/1V-HS.