The power - unit refractive power of the lens or optical system, corresponding to the refractive power of optical glass with a focal length of 1 m is Designated by the letter D. the Strength of glass is inversely proportional to the focal length, i.e., the shorter the focal length, the stronger the glass and on the contrary, the longer the focal length, the glass weaker. For example, glass 10.0 D has a focal length of 10 cm, and the glass in the 2.0 D - 50, see the plus Sign (+) in the number of D indicates that the lens collectively, the minus sign (-) indicates that the lens scattering.
Refractive power of the glass is determined by tiptimer (see Points).