Despite the large maximum aperture, the lens remains compact. Lens group 5 moves for rear focusing, and sharp, crisp pictures are obtained at all apertures. The background blur is ideal for portraits. The USM autofocuses the lens quickly and quietly. The background blur is ideal for portraits 100mm focal length 1:2.0 maximum aperture 247° diagonal angle of view 0.9m / 3 ft. closest focusing distance