Sigma 17-70mm F2.8-4 DC OS HSM is an Auto Focus (AF) Standard Zoom lens with APS-C / DX max sensor coverage. When mounted on a APS-C / DX sensor camera, it will give you an FF equivalent focal range of 25.5 - 105mm. With a max aperture of f2.8, this lens is fast on wide end. The long end is also quite fast at f4.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Sigma 17-70mm F2.8-4 DC OS HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.