One of the middle lens groups in front of the diaphragm moves during focusing resulting in fast AF and light manual focusing. No change in physical length of the lens body which also remains quite small.
A symetrical lens has two identical lens groups on either side of the shutter facing in opposite directions.
The outer lens elements of the two lens groups are concave meniscus lens elements, the inner lens elements are convex meniscus lens elements, made of another glass sort. This lens construction reduces effectively radial distortion.
Two Special Low Dispersion (SLD) glass elements in the front lens group and one in the rear group offer effective control of color aberration. This lens features full-time manual focusing and a removable tripod collar.
These systems use a sensor to recognize camera movement and then compensate for it by shifting a lens group in the lens, or shifting the sensor in the camera. When the camera has a fixed lens, it doesn't matter which approach is used.
A conventional double helicoid focusing system requires that all lens groups be moved by the lens barrel to either the front or rear during focusing.
Rear Focusing System A system in which only the rear lens group moves during focusing, thus eliminating changes in the physical length of the lens during focusing and enabling faster focusing. Such lenses are designated with RF on the lens barrel.
-IF (Internal Focusing) - system in which only the internal lens group shifts during focusing.
The lens employs an inner focusing system in which the first lens group is fixed in position and the second through fourth lens groups move independently in a three-group floating system. The EF-S 60/2.
When focusing, only the rear lens group moves. This eliminates the changing of the physical length of the lens during focusing and allows for faster focusing. (Refer also to Internal Focusing.)
Nikonians Photo Glossary ...
The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus.
INTERNAL FOCUSING - Lens in which internal lens groups shift during focusing so that the external length of the lens does not change.
IF - Internal Focus. Lenses with this designation move the inner lens group or groups without changing the lens' physical length to ensure stability in focusing.
Image-stabilized lenses enable a photographer to shoot at shutter speeds up to about 3stops slower than would normally be possible. They do this by sensing lens movement usinggyroscopic sensors and using the signals to control a lens group which ...
focus is designed longer than the lens' focal length to allow clearance for the movement of the reflex-mirror (No Mirror Lock up or separate viewing accessory attachment is required). It consists of front diverging and rear converging lens groups, ...
earth elements and other minerals in order to alter the refractive properties of the glass to better perform some specific function. Gluing two elements together, often ones that employ different glass compounds than each other, creates a lens group.
Macro focusing, applied to zoom lenses, moves the lens group(s), enabling the lens to focus closer than the normal focusing distance from close-up shooting.
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From the Greek the means "painting or writing with light." ...
 See also: Group, Lens, Lenses, Focus, Camera
  
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