A lens will not focus different colors in exactly the same place because the focal length depends on refraction and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths). The amount of chromatic aberration depends on the dispersion of the glass.
One way to minimize this aberration is to use glasses of different dispersion in a doublet or other combination. Another approach uses spaced doublets.
One way to minimize this aberration is to use glasses of different dispersion in a doublet or other combination. Another approach uses spaced doublets.