Panasonic 100-400mm F4.0-6.3 is an Auto Focus (AF) Telephoto Zoom lens with FourThirds max sensor coverage. When mounted on a FourThirds sensor camera, it will give you an FF equivalent focal range of 200 - 800mm. With a max aperture of f4 at 100mm, we can not consider this lens fast on wide end, and it is slow at the tele end with its max aperture of f6.3.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Panasonic 100-400mm F4.0-6.3 is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body.