Sigmas splash-proof feature is also included preventing dust & precipitation from getting inside the lens. One SLD & two FLD glass elements provides maximum correction of chromatic aberrations. The super multi-layer coating reduces flare & ghosting, & the lens design incorporates an inner focusing & inner zooming system to ensure sharp images throughout the entire zoom range. enhanced optical performance provides excellent correction of sagittal coma flare & ensures ultra-high resolution, comparable to a fixed focal length lens. The HSM (Hyper Sonic Motor) ensures quiet & 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 & having the versatility to zoom in on a players face or hand & then instantly zoom back to capture the entire player making his shot.