Lens Description: The Sigma 150-500mm f/5-6.3 is ultra telephoto zoom designed for Full Frame sensors but may also be used with smaller APS-c size sensors with a corresponding effective increase in focal length to about 255mm to 850mm on the Sigma SD format. An Optical Stabilizer (OS) allows for handheld photography even in low light scenarios, while a Hyper Sonic Motor (HSM) ensures quiet, high speed, and accurate autofocusing. This unique lens is an excellent optical performer featuring three SLD (Special Low Dispersion) glass elements for optimum color correction and sharpness throughout the entire zoom range, rear focusing system corrects for fluctuation of aberration due to focusing. It compact size of a mere 10 inches and weight of only 4 pounds for its focal length make it a must have for any camera bag.