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Fluorite and UD Glass
The refraction of light differs depending on the wavelength. The point of focus therefore differs depending on the different wavelengths or colors. When the different wavelengths are focused at different points, the colors look smeared. This is called chromatic aberration. The longer the focal length, the more pronounced chromatic aberration becomes. Usually, an achromatic element is used in a lens to correct chromatic aberration. However, normal optical glass can only be corrected for two primary spectral colors. An exception to this limitation is Fluorite, an ideal material. Fluorite, which is crystalline, has abnormally low refraction and dispersion characteristics, which optical glass cannot achieve. It also has anomalous dispersion from the green to blue wavelengths. Canon developed production technologies to manufacture fluorite. By incorporating Fluorite in lenses, the points of focus of the three primary spectral colors of red, green and blue all meet at one point for ideal correction of chromatic aberration. There is also UD-glass, which is a special type of optical glass whose properties nearly match those of Fluorite. The effect of two UD-glass elements gives almost the same effect as one Fluorite element. And one super-UD-glass element gives almost the same effect as one Fluorite element
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