The lens system is made up of a Special Low Dispersion (SLD) lens and two aspherical lens, aimed at correcting various aberrations associated with digital cameras, and provide better optical performance in the zoom range. In fact, it's surprising that the lens is so compact, at just 79mm (3.1") length and a diameter of 82.5mm (3.2").

Thanks to the large F2.8 aperture, performance is not degraded even in low-light conditions. A Zoom Lock switch is provided to avoid the zoom creep during transportation.

For more details, check out Sigma's website.