Sigma 120-400mm F4.5-5.6 DG OS HSM is an Auto Focus (AF) Telephoto 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 120 - 400mm. With the max apertures of only f4.5 at 120mm, and f5.6 at 400mm, the lens is quite slow at both ends.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Sigma 120-400mm F4.5-5.6 DG OS HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.