Traditionally, portraits are shot with focal lengths between 50mm and 135mm, though there is no reason you can't use a longer focal length. Nonetheless, 400mm f/2.8 lenses are traditionally reserved for professional sports and wildlife photographers who need a lot of reach and the ability to capture as much light as possible. With super-sharp results and a narrow depth of field, they seem like a great (if somewhat impractical) option for portraits, and this fun video shows you what you can expect.