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.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. 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.