This F2.8 Lens Sets A New Standard For All Zooms. Its Unusually Large Maximum Aperture Is Made Possible By An Extra-wide Aspherical Lens To Provide Higher Optical Performance In A Number Of Ways … Including Effective Anti-flare Features And Crisp-to-the-edge Images. An Internal Focusing Mechanism Allows The Front Of The Lens To Remain Stationary And To Easily Accommodate Specialty Filters, Such As Polarizing Filters. Sigma's Original 4-group Zoom Mechanism Keeps Size And Weight To A Minimum While Fully Correcting Astigmatism And Distortion. Its Wide Range And Convenience Make It Perfect For Snapshots And Great Travel Photos.