Achromatic Doublet Lenses from China tell you that chromatic aberration is caused by the difference in dispersion and refractive index of different wavelengths of light in the glass, resulting in different focal points for different wavelengths of light. Achromatic Doublet Lenses from China tells you that using the combination of different materials, such as the combination of crown and flint glass in the converging lens and diverging lens, the dispersion of each glass can be compensated by the other glass, so that the overall chromatic aberration is reduced to lowest.