Sigma 24mm F1.8 EX DG 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 28mm. Providing a max aperture of f1.8 at 28mm, this lens is very fast.
Sigma 24mm F1.8 EX DG is not a weather sealed lens, so you have to be extra careful against rain and dust.