DESC: Sigma 500mm f4.5 EX Apo is one of the EX Series lenses. It is an Apochromatic ultra-telephoto lens with a fast F4.5 maximum aperture, and 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 and fast auto focus speed.