Canon 400mm f4.0 DO IS USM 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 f4 at 400mm, this is a moderately fast lens.
This lens features on optical image stabilization which helps capturing sharper images at lower shutter speeds. Canon 400mm f4.0 DO IS USM is weather sealed which makes it an ideal lens for outdoor use, especially when coupled with a weather sealed camera body. Canon 400mm f4.0 DO IS USM was replaced with Canon EF 400mm F4 DO IS II USM. Follow the link to read our comparison of these cameras:
Compare Canon EF 400mm f4.0 DO IS USM vs Canon EF 400mm F4 DO IS II USM