Zoom lens
A variable focus lens (also called zoom lens) is a camera lens that can vary its focal length.
In general, such lenses have a lower aperture than comparable fixfocus lenses. Depending on the manufacturer common zoom lenses (in the 30–100mm focal range, for film photography) usually have a varying aperture, from 4 to 5.6, perhaps 3.3 to 5.6. There are also some zoom lenses with a fixed maximal aperture (to take the example from before: of 2.6 or 2.8, maximum). In contrast, fix-focus lenses in that range have maximum apertures of 1.4 to 2.
Digital zoom
Digital cameras sometimes have a function called digital zoom. This is a mathematical method which simply enlarges a part of the picture. The result is not as sharp as that of the optical zoom. When an optical zoom changes its focal length, this will also change the angle of view. Digital zooms simply make a small part of an image bigger, without changing the angle of view.
Zoom Lens Media
Nikkor 28–200 mm zoom lens, extended to 200 mm at left and collapsed to 28 mm focal length at right
TV camera and Canon DIGI SUPER 86 II zoom lens with 86× magnification
Unusual trailed-zoom view of a VLT telescope building
Movement of lenses in an afocal zoom system
Cross section of Fujinon XF100-400mm zoom lens
Canon camera with a 35~70 Template:F/ zoom lens. The advantage of a zoom lens is the flexibility, but the disadvantage is the optical quality. Prime lenses have a greater image quality in comparison.