When the lens moves, the light rays from the subject are bent relative to the optical axis, resulting in a blurred image because the light rays are deflected. By shifting the IS lens group on a plane perpendicular to the optical axis to counter the degree of image shake, the light rays reaching the image plane can be steadied.
Figure shows what happens when the there is a downward movement with the lens.
The center of the image moves downward on the image plane. When the IS lens group shifts on the vertical plane, the light rays are refracted so that the image center returns to the center of the image plane. Since image shake occurs in both horizontal and vertical directions, the IS lens group can shift horizontally and vertically on a plane perpendicular to the optical axis to counteract the image shake. Two shake-detecting sensors, one each for yaw and pitch, are used to detect lens movement. The microcomputer then converts the detection signals into drive signals for the IS lens group.