Sigma's APO 800mm f 5.6 EX is one of the EX Series lenses. It is an Apochromatic ultra-telephoto lens with a fast F5.6 maximum aperture, & incorporates two Extraordinary Low Dispersion glass elements (Abbe number 90.3), to reduce chromatic aberration to minimum limit. It also incorporates internal focusing which enables responsive & fast auto focus speed. The AF drive for Sigma SA, Canon & Nikon S.L.R. cameras features the ultra-quiet Hyper Sonic Motor (H.S.M.). This new lens also incorporates a rotary filter holder in the rear part of the lens barrel, which enables the use of a polarizing filter. Sigma's dedicated APO EX Tele Converter 1.4x MF & 2x MF can be used with this lens. The lens materials used in this new lens are lead & arsenic free ecological glass.