Before zoom lenses photographers used prime lenses. The angle of view for a prime lens was fixed, sometimes known as a fixed focal length lens. A new film camera would have been supplied with a standard lens with an angle of view similar to our eye sight. On more advanced cameras the photographer would have the facility to remove the standard lens, replacing it with either a telephoto lens (again of fixed focal length), or a wide-angle.

With the introduction of zoom lenses the restrictions imposed by a prime lens were removed. It became possible to change the field of view continuously between two limits dictated by its magnification and expressed as a number - e.g. 3x or 5x. Most common was a lens that started its zoom ratio as a modest wide-angle with a low focal length number, passing through standard to a modest telephoto with a much higher focal length number. This made composition more convenient, such as removing that wastepaper bin on the edge of the picture.

These optical zoom lenses (not to be confused with digital zoom) have different rates of magnification, some covering the telephoto end of the spectrum, others wide-angle. Unfortunately unlike film lenses, digital does not have a standard to allow an easy comparison, but generally a focal length with a high number will give a pronounced telephoto effect, whereas a focal length with a low number is wide-angle.