ABSTRACT
Most people who will at some time have used a lens to focus an image of the sun or some bright object know the principles of a simple lens. At the same time they will have observed that as the distance from the object to the lens is changed, the image focuses at a different distance and the size of the image changes. This principle of magnification is used in the production of a zoom lens. In a zoom lens the focal length can be changed continuously to alter the magnification whilst the image still remains in focus.
