General optical systems thumb|upright=1.5|Thick lens diagram For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses or mirrors (e.g. a photographic lens or a telescope), there are several related concepts that are referred to as focal lengths: Effective focal length (EFL) The effective focal length is the inverse of the optical power of an optical system, and is the value used to calculate the magnification of the system. The imaging properties of the optical system can be modeled by replacing the system with an ideal thin lens with the same EFL. The EFL also provides a simple method for finding the nodal points without tracing any rays. It was previously called equivalent focal length (not to be confused with 35 mm-equivalent focal length). Front focal length (FFL) The front focal length is the distance from the front focal point to the front principal plane . Rear focal length (RFL) The rear focal length is the distance from the rear principal plane to the rear focal point . Front focal distance (FFD) The front focal distance (FFD) () is the distance from the front focal point of the system () to the vertex of the first optical surface (). Some authors refer to this as "front focal length". Back focal distance (BFD) Back focal distance (BFD) ({{math|''}}) is the distance from the vertex of the last optical surface'' of the system () to the rear focal point (). Some authors refer to this as "back focal length". For an optical system in air the effective focal length, front focal length, and rear focal length are all the same and may be called simply "focal length".
General optical systems thumb|upright=1.5|Thick lens diagram For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses or mirrors (e.g. a photographic lens or a telescope), there are several related concepts that are referred to as focal lengths: Effective focal length (EFL) The effective focal length is the inverse of the optical power of an optical system, and is the value used to calculate the magnification of the system. The imaging properties of the optical system can be modeled by replacing the system with an ideal thin lens with the same EFL. The EFL also provides a simple method for finding the nodal points without tracing any rays. It was previously called equivalent focal length (not to be confused with 35 mm-equivalent focal length). Front focal length (FFL) The front focal length is the distance from the front focal point to the front principal plane  Rear focal length (RFL) The rear focal length is the distance from the rear principal plane to the rear focal point . Front focal distance (FFD) The front focal distance (FFD) () is the distance from the front focal point of the system () to the vertex of the first optical surface (). Some authors refer to this as "front focal length". Back focal distance (BFD) Back focal distance (BFD) ({{math|''}}) is the distance from the vertex of the last optical surface'' of the system () to the rear focal point (). Some authors refer to this as "back focal length". For an optical system in air the effective focal length, front focal length, and rear focal length are all the same and may be called simply "focal length".