Sigma 500mm f 4.5 EX Apo is one of the EX Series lenses. It is an Apochromatic ultra-telephoto lens with a fast F4.5 maximum aperture, & it incorporates two Extraordinary Low Dispersion glass elements (Abbe number 90.3), to reduce chromatic aberration to a minimum. It incorporates internal focusing which enables responsive & fast auto focus speed. The AF drive for Sigma SA, Canon & Nikon S.L.R. cameras features Sigma's ultra-quiet Hyper Sonic Motor (H.S.M.). It also features a rotatable drop-in filter holder in the rear part of the lens barrel, which facilitates the use of filters, including a polarizing filter. Sigma's dedicated APO Tele Converter 1.4x MF & 2x MF can be used with this lens (in case of Canon EOS3 the 1.4x teleconverter can also provide the AF function). The lens materials used in this new lens are lead & arsenic free ecological glass.