Sigma 135mm F1.8 DG HSM is an Auto Focus (AF) Telephoto 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 135mm. Providing a max aperture of f1.8 at 135mm, this lens is very fast.
Sigma 135mm F1.8 DG HSM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.