Zoom lens, telephoto lens and retrofocus lens

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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.
Parfocal lens and varifocal 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).
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A parfocal lens design
Applications Video, motion picture cameras
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A varifocal lens design
Application still camera
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Telephoto lens system
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Single-lens reflex camera
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Shrink system length
Schmidt-Cassegrain-Telescope
Maksutov-Cassegrain-Telescope
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Retrofocus lens system
retrofocus
Wide field lens system