<P> The distance from the front focal point of the combined lenses to the first lens is called the front focal length (FFL): </P> <Dl> <Dd> FFL = f 1 (f 2 − d) (f 1 + f 2) − d . (\ displaystyle (\ mbox (FFL)) = (\ frac (f_ (1) (f_ (2) - d)) ((f_ (1) + f_ (2)) - d)).) </Dd> </Dl> <Dd> FFL = f 1 (f 2 − d) (f 1 + f 2) − d . (\ displaystyle (\ mbox (FFL)) = (\ frac (f_ (1) (f_ (2) - d)) ((f_ (1) + f_ (2)) - d)).) </Dd> <P> Similarly, the distance from the second lens to the rear focal point of the combined system is the back focal length (BFL): </P>

Magnification of a lens is also known as which of the following