effective focal length
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effective focal lengthThe property of a compound lens, such as an eyepiece, that is analogous to the focal length of a single thin lens and can be used, for example, in the determination of magnifying power. For a two-lens system the effective focal length, f , is given by
1/f = 1/f 1 + 1/f 2 – d /f 1 f 2
where f 1 and f 2 are the focal lengths of the components and d is their separation.