Sigma 2x EX DG Tele Converter The 2x EX is placed in the same way and it allows the photographer to use the 70-200mm f/2.8 lens as a 140-400mm f/5.6 telephoto zoom lens, and the 300mm f/2.8 lens as a 600mm f/5.6 ultra telephoto lens. Sigma APO Tele Converters are dedicated to Sigma lenses such as the APO 70-200mm f/2.8 EX DG and APO 300mm f/2.8 EX DG. They increase the focal length by 1.4 or 2 times. By providing a longer focal length, they allow greater flexibility, which is useful for taking sports of nature photographs.