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How do zoom lenses differ from lenses with a fixed focal length?

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Diagram showing the negative-positive photographic process: a scene is captured on film in a camera, developed into a negative, projected onto paper with an enlarger, and developed again to produce a positive print. — technology of photography Figure 1: Sequence of negative–positive process, from the photographing of the original scene to enlarged print (see text).

In the technology of photography, the primary difference between a zoom lens and a lens with a fixed focal length lies in the ability to adjust the focal length during use.

Zoom Lenses: These are complex optical systems, often containing 12 to 20 elements, that allow the user to vary the focal length continuously while the image remains in focus. By moving internal lens elements or groups via levers or cams, a single zoom lens can replace a range of interchangeable lenses, providing flexibility in image scale and field of view without requiring the photographer to change equipment.12
Fixed Focal Length Lenses: These lenses have a single, unchanging focal length. To alter the image scale or the amount of the scene captured, photographers typically must use different interchangeable lenses, each designed for a specific focal length.3

While a true zoom lens maintains focus throughout its range, some varifocal lenses may require manual refocusing when the focal length is changed. Because of the need to correct aberrations across a wide range of focal lengths, zoom lenses are generally more complex in their construction than fixed lenses.21