Sigma 12-24mm F4.5-5.6 II DG HSM is an Auto Focus (AF) Wideangle Zoom lens with 35mm FF max sensor coverage. When mounted on a 35mm FF sensor camera, it will give you an FF equivalent focal range of 12 - 24mm. With the max apertures of only f4.5 at 12mm, and f5.6 at 24mm, the lens is quite slow at both ends.
Sigma 12-24mm F4.5-5.6 II DG HSM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body. Sigma replaced the older Sigma 12-24mm F4.5-5.6 EX DG Aspherical HSM Sigma SA with this model. Follow the link to read our detailed comparison of these model: Compare Sigma 12-24mm F4.5-5.6 II DG HSM Sigma SA vs Sigma 12-24mm F4.5-5.6 EX DG Aspherical HSM Sigma SA