This Canon Extender 2x is an ultra high performance teleconverter for use with Canon's high-performance telephoto lenses on all Canon EOS digital and 35mm cameras. It multiplies your focal length by 2x and makes your effective aperture two stops less. It makes your 70-200/2.8 into a 140-400mm f/5.6, or your 300mm f/2.8 into a 600mm f/5.6. It works with my 70-200/4 IS, making it a 140-400mm f/8.description