An aperture stop control device for a zoom lens system includes a variable aperture stop, a first sensor for determining the diameter of the aperture stop and providing a first electrical signal representative of the diameter of the aperture stop means and a second sensor for generating a second electrical signal indicative of a designated F number. A signal generator provides a third electrical signal indicative of the focal length of the zoom lens system. A computing circuit computes an output electrical signal as a function of the second and third electrical signals to control the diameter of the aperture stop for automatically maintaining a designated F number during a zooming operation.