<Dd> f ≈ v . (\ displaystyle f \ approx v \ .) </Dd> <P> 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), the focal length is often called the effective focal length (EFL), to distinguish it from other commonly used parameters: </P> <Ul> <Li> Front focal length (FFL) or front focal distance (FFD) (s) is the distance from the front focal point of the system (F) to the vertex of the first optical surface (S). </Li> <Li> Back focal length (BFL) or back focal distance (BFD) (s ′) is the distance from the vertex of the last optical surface of the system (S) to the rear focal point (F ′). </Li> </Ul> <Li> Front focal length (FFL) or front focal distance (FFD) (s) is the distance from the front focal point of the system (F) to the vertex of the first optical surface (S). </Li>

What is the focal length of a lens