It is an Apochromatic ultra-telephoto lens with a fast F5.6 maximum aperture, and 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 and fast auto focus speed. ELD Glass elements ensure high quality images Incorporating two Extraordinary Low Dispersion glass elements to reduce chromatic aberration to a minimum and ensure sharp, quality images. HSM The HSM models (Sigma, Nikon and Canon mounts) provide high-speed, ultra-quiet AF function as well as full- time manual focus override. It features a drop-in filter This new lens also supplied with a 46mm gelatin filter holder at the rear, allowing the use of gelatin filters. Inner focusing Inner focusing retains external dimensions for ease of handling. Optional Sigma tele-converter By adding Sigma?s dedicated APO EX Tele Converter 1.4x and 2x, you can use this lens as a 1120mm F8 MF, or a 1600mm F11 MF lens respectively.