True Zoom Telephoto Lens

In a true zoom lens, two or more of the lens elements are moved within a fixed tube to change the focal length while keeping the image in focus on the film. One scheme with the triplet arrangement shown below is to keep the front and back elements at a fixed separation and move them and the center element to accomplish the zoom.

If the front lens focal length =
middle lens focal length =
and back lens focal length = ,
the equivalent focal length is
. Front to back lens distance =
Middle to back lens distance =
Back lens to film distance =
Calculation to be added.
Example
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Camera concepts

Optical instruments
 
HyperPhysics***** Light and Vision R Nave
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Discussion of principles
Index

Camera concepts

Optical instruments
 
HyperPhysics***** Light and Vision R Nave
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