Sigma's splash-proof feature is also included preventing dust and precipitation from getting inside the lens. One SLD and two FLD glass elements provides maximum correction of chromatic aberrations. The super multi-layer coating reduces flare and ghosting, and the lens design incorporates an inner focusing and inner zooming system to ensure sharp images throughout the entire zoom range. Improved optical performance provides excellent correction of sagittal coma flare and ensures ultra-high resolution, comparable to a fixed focal length lens. The HSM (Hyper Sonic Motor) ensures quiet and high speed AF, while allowing full-time manual focus override. A truly unique product which is a rarity in the photo industry, this lens is the only lens of its kind on the market; a professional zoom lens that also features a fast aperture. Most professionals own the standard 300mm 2.8, but they also have to carry a shorter zoom lens for those times when the subjects move closer. That means fumbling around switching lenses or carrying a second camera body. The amazing Sigma zoom lens let you instantly determine the picture composition you want without having to change your vantage point. Imagine photographing courtside at a college basketball game and having the versatility to zoom in on a player's face or hand and then instantly zoom back to capture the entire player making his shot.