Sony FE 400mm F2.8 GM OSS 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 400mm. With a max aperture of f2.8, this is a fast lens.
Sony FE 400mm F2.8 GM OSS is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.