Sigma 17-50mm F2.8 EX 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 - 75mm. With a max aperture of f2.8, this lens is fast on wide end and equally fast at 50mm with a max aperture of f2.8.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Sigma 17-50mm F2.8 EX DC OS HSM is not a weather sealed lens, so you have to be extra careful against rain and dust.