The focal length of a compound optical system, given as if it were a single optical element, is called the equivalent focal length. The focal length at which an optical system seems to be working in a given situation is known as the effective focal length. For example, if an eyepiece projects an image onto a film, the image is magnified as if it were made by a telescope of several times the telescope's equivalent focal length. The back focal length is the distance between the last surface of a compound optical system and the focal plane of the system and may be quite different from the actual focal length.
Related category OPTICS AND OPTICAL PHENOMENA
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