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How does a zoom lens change focal length during a shot?

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Director Steven Spielberg and camera operator Michael Chapman film next to a large mechanical shark on the set of the 1975 movie Jaws. — motion-picture technology Director Steven Spielberg holding the movie camera, assisted by camera operator Michael Chapman, during the filming of *Jaws* (1975).

A zoom lens changes its focal length during a shot by physically moving one or more internal lens elements or groups along the optical axis.123

In a typical design, this is achieved by using a system of multiple lens elements—sometimes as many as 18 to 20—arranged in groups. By adjusting the separation between these groups using levers, cams, or an electric motor, the lens can continuously vary its focal length.132

For example, a simple configuration may involve a movable concave (diverging) lens positioned between two fixed convex (converging) lenses. As the central element moves closer to the front or rear convex element, the combined focal length of the system changes, which alters the scale of the image. In a true zoom lens, this movement is precisely calibrated so that the image remains in focus throughout the entire range of the zoom.243