From Chrysis.net
LINKY
IS (Image Stabilizer) System
One of the causes of a photography failure is always has been the "moved", so the only alternative to a tripod was the use of more sensible films or the resource to a fortune support. Canon therefore has developed an image stabilizing system, using it either in the telecameras and inserting it into the lenses. The vibrations transmitted from the hand to the camera transmit also to the lens and therefore to the optical elements. The lens movement causes the movement of the image formed, regarding the optical axis. If, therefore, the elements could move independently from the system, the problem would be resolved. This idea has been the assumption to the development of Canon's stabilizing system.
The system is based on the connection of two vibrating gyroscopes that detect the vertical and horizontal vibrations and command an equal and contrary reaction of the sophisticated optical group assigned to the correction. The answer time of the correction is in the order of 1/2,000 of a second. It turns out a corrective effect equal to two stops in terms of exposure: the gain is therefore of two stop (equivalents to a 2 stop more luminous lens or to a 2 stop faster film) before a really moved image. The stabilization system is useful also in the panning technique (you follow a moving subject moving your camera). The software detects the movement typical of the technique and switches off the correction in the direction of the panning, keeping on only the correction of the vibration in the other directions.
IS Mode 2: this new technology automatically determines that the movement is conscious panning and halts correction in the relevant direction, thereby preventing shake-return. The IS Mode 2 system automatically detects lack of movement / vibration to prevent operation errors when a tripod is in use. IS efficacy increases and lower overall size have been achieved through improvements in the moving contact sections of the IS lens barrel, the IS lens group barrel locking mechanism and the shake-detection gyro.
My two IS lenses 'crackle' very faintly when switched on!
Hope this helps.
Baz.