equivalent focal length

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length, equivalent focal

In an optical system composed of more than one lens, it is the linear distance separating the principal focus from the corresponding principal point. It is usually the most important quantity in the specification of an optical system as in objectives, eyepieces, etc. See principal focus; equivalent power.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann
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