General photographic lenses carry out focusing using either the all-group focusing, in which all lens groups are moved together along the optical axis, or the front-group focusing, in which only front lens group is moved. Those focusing systems require much energy for focus driving mechanism, thereby preventing faster auto focusing. Such focusing systems are also difficult to make the lenses compact due to large front lens groups. The rear and inner focusing systems are developed mainly to reduce the weight of moving focusing lens group. The rear and inner focusing systems employ, as a focusing lens group, optical systems other than the front lens group. Canon adopted those systems in super telephoto lenses as early as 1975. In the EF lens series, the systems are used in telephoto lenses as well as zoom and wide angle lenses. Canon uses in-house terms such as "inner focusing" meaning lens group(s) between front lens and diaphragm controls focusing, and "rear focusing" meaning lens group(s) behind diaphragm controls focusing.
In addition to speeding up autofocusing and downsizing the lens, the rear and inner focusing systems have the following benefits;
1. easy handling of the lens because of unchanged total length of the lens during focusing.
2. easier to shorten the minimum shooting distance compared to the all-group focusing and front-group focusing methods.
3. easier operation of the polarizing filters because of non-rotating front frame.
