There are several methods of moving of optical elements to carry out focusing:
All-group focusing All lens groups are moved forward or backward. It is the simplest method of focusing used mostly in wide-angle and standard prime lenses. The advantage of all-group focusing is that the level of aberrations remains more or less the same with changing of focusing distance. However such method is less convenient to use with telephoto and supertelephoto lenses due to increase of size and weight of the optical system and large angle of rotation of manual focusing ring which complicates precise manual focusing with fast moving subjects.
Front group linear extension. Only the front lens groupd moves forward or backward during focusing and the rear group remains static. Such method of focusing is used in some macro lenses.
Front group extension with rotation. A part of housing around the front lens group rotates during focusing in order to move the front group forward or backward. Such method of focusing is only used in zoom lenses and allows to design simple optical schemes but limits the zoom factor and does not allow to design compact optical schemes.
Internal and rear focusing was introduced in order to overcome weak points of focusing methods mentioned above. The optical system is «split» in several parts. Only several groups of elements in front of the aperture (in case of internal focusing) or behind the aperture (in case of rear focusing) move relative to each other during focusing.
Internal and rear focusing are now used by all major manufacturers of photographic lenses and not only in telephoto and supertelephoto lenses but also in wide-angle and macro lenses. Lenses with internal or rear focusing usually have «IF» designation in their names.
The lenses with internal or rear focusing have the following advantages:
- Since the only one lightweight group of elements moves during focusing, the manual focusing can be performed easily and the energy effeciency of the autofocus is on a high level;
- The overall length of the lens remains unchanged during focusing which allows to design compact and rigid lenses;
- The manual focusing ring can be placed in the optimal part of the housing and it won’t be moving forward or backward during focusing;
- The minimum focusing distance is lesser compared with lenses which use all-group focusing or front group linear extension;
- The filter thread does not rotate during focusing which makes it convenient to use polarizing or gradient filters;
- A part of the housing around the front lens group does not rotate or move during focusing which allows to use petal (flower) lens hood instead of round to provide more efficient protection of the front element from flares or damage.
Internal or rear focusing is especially useful when shooting with macro lenses because it reduces the risk that the front element of the lens will accidentally touch the subject or cast a shadow on it.
As the floating system, the internal or rear focusing has its shortcoming: the actual focal length of the lens decreases with decreasing of focus distance and the field of view becomes wider.