Sigma 170-500mm F5-6.3 APO Aspherical AutoFocus Telephoto Zoom                         

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 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.
Specifications
Construction 11 groups /13 elements
Angle of view 14.5°-5.0°
F stop range 32-38
Closest Focusing Distance 300-320cm / 118.1-126"
Maximum Magnification 1:6.6
Filter Size 86mm C
Dimensions (Length x Diameter) 92.5 x 229.5mm / 3.6 x 9.0"
Full Extended Length 315.0mm / 12.4"
Weight 1,320g / 46.6oz.