There are many different designs for zoom lenses, but they all have some basic principals in common. They consist of a number of differently shaped individual lenses or â€˜elementsâ€™, some of which move relative to one another to alter the magnification of the image without altering the focus. The diagrams below illustrate a very simplified design for a typical zoom lens. It consists of two distinct lens systems, the zoom system and the focusing elements. The key element of the zoom system is a concave lens which disperses the light path, and which can be moved relative to a convex lens behind it which gathers it in again. The function of the zoom system is simply to control the width or dispersal of the light rays entering the front of the lens, and therefore change the magnification. The zoom system does not focus the light. This job is done by the rear elements of the lens system which focus the rays onto the imaging sensor ensuring a sharp picture.
In this first diagram, we can see the relative positions of the lens elements when the lens is set to wide angle. As you can see, the path of the light entering the front element of the lens is narrowed, producing a lower magnification. As a side effect, this also concentrates the light entering the lens, allowing it all to fall on the sensor, which is why wide-angle settings have a larger effective aperture value.
In the second diagram the lens is set to telephoto. The concave element disperses the light path, so only the centre area of it is gathered by the rear element of the lens. This produces higher magnification, since only the centre potion of the image is captured by the sensor. The dispersal of some of the light entering the lens is why longer focal lengths have a narrower effective aperture value.