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a combination of a telescopic attachment and an objective lens, forming an optical system with a variable focal length. The mechanical motions of the individual elements of the telescopic attachment ensure that the image scale of an object varies smoothly within a specified range. The focal length of a zoom lens is varied, but the sharpness of focus of the objective and the relative aperture remain unchanged.
Zoom lenses are used most often on motion-picture cameras for creating the illusion that the camera is approaching or receding in cases where it is not desirable to move the camera relative to the object being filmed. Because the aberrations of the telescopic attachment and the objective lens are usually calculated separately, the same attachment may be used with different objective lenses.
L. A. RIVKIN