For the first time in optical history, this ultra-zoom lens employs a seven-group zoom system, which provides a high magnification. This system ensures a superior level of precision from 50mm standard to 500mm ultra focal length range.In addition, by utilizing triple-cam Rear Focusing System effectively, high precision is maintained throughout the whole focus length.By attaching the 2X APO EX Tele-Converter, this lens can be used as a 200-1000mm/F10.4-12.6 manual focus zoom lens, while maintaining a very compact size for 1000mm focal length. It is also possible to use AE (Automatic Exposure) function, when EX Tele-converters are attached.An apochromatic lens, which keeps color aberration to a minimum, using Special Low Dispersion (SLD) or Extraordinary Low Dispersion (ELD) glass is called an APO series lens at Sigma.This lens uses four pieces of SLD (the special low dispersion) glass for excellent correction of chromatic aberration. High optic performance is demonstrated throughout the entire zoom range.The HSM models (the HSM is only available for AF Canon, Nikon and Sigma Mounts) provide quiet high-speed AF function, as well as full time manual focus capability.This lens also features a Zoom Lock device to eliminate "Zoom Creep" when the lens is tilted down. When the lens is set to the 50mm focal length, it is remarkably compact and provides easy handling. The tripod mount is composed of magnesium, to minimize total weight and also it is easy to grip and carry the lens due to ergonomic design of the tripod mount.Non-rotating front lens elements, due to rear focus mechanism make possible the use of a custom bayonet "Petal-type Hood", which prevents extraneous light from degrading the image quality. This mechanism also allows the use of circular polarizing filters, since the front barrel does not rotate during focusing or zooming.