Sigma 20mm F1.4 DG HSM A is an Auto Focus (AF) Wideangle Prime 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 20mm. Providing a max aperture of f1.4 at 20mm, this lens is very fast.
Sigma 20mm F1.4 DG HSM A is not a weather sealed lens, so you have to be extra careful against rain and dust.