This great lens combines a 5 lens group zoom system with an aspherical lens element to deliver sharp high contrast images throughout the entire zoom range. Using Sigma's unique apochromatic lens design and three SLD glass elements, the aberrations usually found in ultra-telephoto lenses have been eliminated. And with Sigma's new focusing system, the front lens group doesn't rotate, which makes using polarized filters a snap. A convenient built-in rotary tripod mount eliminates vibration during shutter release.