Lens Focusing Systems.

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.
The 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.

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