effective focal length


effective focal length

The 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 2d /f 1 f 2

where f 1 and f 2 are the focal lengths of the components and d is their separation.