 If the surrounding medium is not air, then the distance is multiplied by the refractive index of the medium ("n" is the refractive index of the substance from which the lens itself is made; "n"1 is the refractive index of any medium in front of the lens; "n"2 is that of any medium in back of it).Some authors call these distances the front/rear focal "lengths", distinguishing them from the front/rear focal "distances", defined above.In general, the focal length or EFL is the value that describes the ability of the optical system to focus light, and is the value used to calculate the magnification of the system.