Extending the lens (focusing lens group) for focusing usually degrades the image quality at close focusing distance. For a wide angle lens, in particular, the extension of the focusing lens group causes curvature of field, a phenomenon where focus is slightly different between center of the film plane and peripheral part due to increasing aberration caused by lens extension. The floating system is developed to correct such fluctuation of aberration, moving a part of the optical system separated from the focusing lens group. The floating system is mainly applied for wide angle lenses. In the EF lens series, however, it is also used in many type of lenses, such as large aperture lenses, middle telephoto lenses and macro lenses, to upgrade the picture quality taken at all shooting range, from the shortest focusing distance to infinity. This mechanism is named "floating", because an optical part moves like a float, not in union with the rest of focusing lens group.
