Thorough elimination of the se
EN: Thorough elimination of the se
When a convex fluorite lens is combined with a concave wide-dispersion optical glass lens to correct red and blue wavelengths, the partial dispersion characteristics of the fluorite also effectively compensate the green wavelength as well. This greatly reduces the presence of secondary spectrum and brings all three wavelengths ~ red, green and blue ~ together at the same focal point to realize virtually ideal chromatic aberration compensation, (apochromatic performance).
Canon glossary EU