Sigma 50-100mm F1.8 A is an Auto Focus (AF) Telephoto 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 75 - 150mm. Providing a max aperture of f1.8 at 50mm, this lens is remarkably fast on wide end. Even better, having an aperture of f1.8 on tele end, this lens is exceptionally fast through out its range.
Sigma 50-100mm F1.8 A is not a weather sealed lens, so you have to be extra careful against rain and dust.