This lens is a monster - it weighs over 13 pounds, is 325mm long, and has a front element 143mm in diameter! Its combination of extreme telephoto length and large aperture makes it great for birding and sports shooting. Because of its large aperture it can comfortably used with various tele-extenders such as the Pentax autofocusing F1.7x or the MF A1.4xL and A2.0xL to get even closer to the subject. I was fortunate to find a used copy in excellent condition at an "affordable" price.