By moving groups of lenses along the optical axis of a lens system, it is possible to continuously vary the focal length of the system, which results in a varifocal system. A zoom lens results whenever the position of the image plane remains stationary. As the focal length of the lens changes, the field of view also changes. The focal length is varied by moving axially at least one group of lenses, called the variator. To maintain the image plane position as stationary, another group of lenses is required, which is called the compensator. The axial movement of the variator and the compensator are usually different in nature. The variator might be moved in a linear manner, and the compensator in a non-linear manner by using a mechanical cam. As shown in Figure 18.1, zoom lenses that maintain the image position by moving the variator and the compensator equally are referred to as having optical compensation. Zoom lenses that require different movements for the variator and the compensator are referred to as mechanically compensated. The variator and the compensator constitute the lens kernel of the zoom lens. Mechanically compensated lenses have more optical design freedom than optically compensated lenses, and most modern zoom lenses are of the former class.