made from bacd5 the refractive index of bacd5 is 1 589 and its dispersion coefficient is 61 25 the second lens element 30 is advantageously made of optical plastic since optical plastic can be more readily shaped machined into the desired complex shape desired for the second lens element a refractive index and a dispersion coefficient of the second lens element 30 need to satisfy the following requirements 1 65 lt n lt 1 65 20 lt v lt 30 the second lens element 30 is preferably made from okp4 the refractive index of okp4 is 1 609 and its dispersion coefficient is 26 64 referring to fig2 the first aspheric surface 32 of the second lens element 30 is a diffractive surface the diffractive surface is toward the object side and is substantially serrated the serrated diffractive surface may advantageously be engraved in the first aspheric surface 32 of the second lens element 30 by means of cutting tools the largest serration depth is about 0 967 microns and the smallest distance of a serration from the optical axis is about 26 7 millimeters the diffractive surface is designed according to optics principles because the light rays are made up of different wavelengths of light the diffractive surface may change an image phase of different wavelength of light so as to.