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.
From the Manufacturer
Another option in high-speed moderate telephotos, this compact lens is ideal for portraits and low-light work. Compared to the 85mm f/1.8 USM lens, it provides that little bit extra of telephoto power sometimes necessary outdoors or for candid shooting. Like the 85mm, itís super-sharp (even wide-open), and has fast USM autofocus along with full-time manual focusing.
A medium telephoto lens with a large aperture brings the subject closer, creates excellent background blur, gives a longer flash range, and affords a faster shutter speed to freeze the action. A standard zoom lens, with its natural angle of view and perspective, captures the subject plainly, with no special effects. However, you can use standard lenses in creative ways by varying the subject distance, aperture, and angle.