Dispersion, or spectral variations in index of refraction, results in differences of focal distances for light of different wavelengths. This means that even though a lens will transmit a particular wavelength, if it was not designed at that wavelength then the performance will not be the same as that stated for the design wavelength. The Abbé number (vd) quantifies the amount of dispersion for a particular frequency range. This defines how much index changes with wavelength and the smaller the value means the quicker the change; vd=(nd-1)/(nF-nC), where nF=486.1nm and nC=656.3nm. Glasses are typically defined as either crowns or flints. Crown glasses have the following combination of values: nd<1.6 and vd>55 or nd>1.6 and vd>50. Flints define the rest and are typically referred to as high index glass.
Glass Index
Glass Index and Abbé Number values are the most important criteria in comparing one material to the next. The index of refraction is actually a ratio of the speed of light in a vacuum to that of light in a medium (i.e., a specific type of glass). Since the speed of light in any glass varies with the wavelength of light, the index of refraction also changes with wavelength. Typically, a glass is defined at nd, which is the index at yellow helium or 587.6nm.