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Zoom lens, telephoto lens and retrofocus lens
A zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length (and thus angle of view), as opposed to a fixed focal length (FFL) lens.
A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes. A lens that loses focus during zooming is more properly called a varifocal lens (see also below).
Parfocal lens and varifocal lens:
A parfocal lens design
Applications: Video, motion picture cameras
A varifocal lens design
Application: still camera
Telephoto lens system
Single-lens reflex camera
Maksutov-Cassegrain-Telescope
Schmidt-Cassegrain-Telescope
Shrink system length
Retrofocus lens system
Wide field lens system
retrofocus
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