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Historically, lenses used in photography where lens with a fixed focal length
The most popular design for manual zoom lenses is the push pull zoom. It uses one control ring for both focusing and focal length changes. You twist the lens focus and push-pull it to change the focal length. Such a simple purely mechanical control also means the lens must rely on a simple optical principle to work.
Most zooms work with a combination combination of 3 lens groups:
1. a focusing group, creating a virtual image of the wanted focal plane at a given position
2. An afocal lens system group which only magnifies the virtual image, without changing the focus
3. A fixed lens to focus the virtual image on the sensor
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