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Special lenses, known as fluorite lenses, and boasting very low dispersion of light, have been developed to resolve the issue of chromatic aberration. Fluorite is actually calcium fluoride (CaF2 ), crystals of which exist naturally. Towards the end of the 1960s, Canon developed the technology for artificially creating fluorite crystals, and in the latter half of the 1970s we achieved the first UD (Ultra Low Dispersion) lenses incorporating low-dispersion optical glass. In the 1990s, we further improved this technology to create Super UD lenses. A mixture of fluorite, UD and Super UD elements are used in today's EF series telephoto lenses.
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