The focal length at which an optical system appears to be working. For example, when the eyepiece of a telescope is used to project an image on to film, the image is magnified as if it were produced by a telescope of several times the telescope’s actual focal length. The magnification is given by (d/F) − 1, where d is the distance between the film plane and the field stop within the eyepiece, and F is the eyepiece’s focal length. The focal length is increased by this factor.