Canon 200-400mm f4L IS USM Extender 1.4x is an Auto Focus (AF) Telephoto Zoom 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 200 - 400mm. With a max aperture of f4 at 200mm, we can not consider this lens fast on wide end but f4 at 400mm is not too bad for tele end.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 200-400mm f4L IS USM Extender 1.4x is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.