What is APO ? An Apochromat lens, which keeps color aberration to a minimum, using Special Low Dispersion glass is called an APO series lens at Sigma. As the refractive index of glass depends on the wavelength of the light, color aberration occurs when different colors form images at different points. This problem often occurs with telephoto lenses. In the case of normal optical glass it can only be corrected for two primary spectral colors by combination of low dispersion convex and high dispersion concave lenses. Sigma Apo lenses use SLD (Special Low Dispersion) or new ELD (Extraordinary Low Dispersion) glass to offer superior sharpness, high contrast and color correction by minimizing the chromatic aberration. What does EX Lens mean ? The Excellent features of these Sigma lenses such as new optical and mechanical design concept, superior performance, perfect handling, ultra compact design, durability etc., are symbolized by the EX mark. These professional type lenses are distinguished by their the distinctive EX finish and EX logo on the lens barrel. What is DG? These are large-aperture lenses with wide angles and short minimum focusing distances. With an abundance of peripheral illumination, they are ideal lenses for Digital SLR Cameras. What does DC stand for? These are special lenses designed so that the image circle matches the smaller size of most digital SLR cameras. Their specialized design gives these lenses the ideal properties for digital cameras, including compact and lightweight construction. What is HSM? HSM stands for "Hyper Sonic Motor". The HSM motor is driven by ultrasonic waves and it makes auto focusing, precise, silent and high-speed. Sigma's HSM version lenses are available only for AF Sigma, Canon and Nikon AF cameras. What is OS? OS stands for “Optical Stabilizer”. OS function utilizes a built-in mechanism that compensates for camera shake. It dramatically expands, photographic possibilities by alleviating camera movement when shooting by hand held camera.