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Lifted from the Canon website - talking about the 500 / 600 f4 IS L
"One fluorite element and two UD-glass elements effectively correct aberrations resulting in sharp and excellent delineation."
Some other reading from Canon
Fluorite
Fluorite has extremely low indexes of refraction and dispersion compared to optical glass and features special partial dispersion characteristics (extraordinary partial dispersion), enabling virtually ideal correction of chromatic aberrations when combined with optical glass. This fact has long been known, and in 1880 natural fluorite was already in practical use in the apochromatic objective lenses of microscopes. However, since natural fluorite exists only in small pieces, it cannot be used practically in photographic lenses. In answer to this problem, Canon in 1968 succeeded in establishing production technology for manufacturing large artificial crystals. Thus opening the door for fluorite use in photographic lenses.
Super UD lenses
The high cost of synthetic fluorite crystal production makes fluorite lenses extremely expensive. One answer was found in the latter half of the 1970s with the appearance of UD (ultra low dispersion) glass that could provide characteristics similar to fluorite but at a lower cost. While the indexes of refraction and dispersion of UD glass do not equal that of fluorite, they are significantly lower than those of other types of optical glass. Moreover, UD glass does display partial dispersion characteristics similar to fluorite. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two UD lens elements are equivalent to one fluorite element). Another breakthrough was made in 1993 when Super UD glass was introduced as a new material that achieves almost the same performance as fluorite while achieving a new balance of greater cost reduction and even higher quality.
"One fluorite element and two UD-glass elements effectively correct aberrations resulting in sharp and excellent delineation."
Some other reading from Canon
Fluorite
Fluorite has extremely low indexes of refraction and dispersion compared to optical glass and features special partial dispersion characteristics (extraordinary partial dispersion), enabling virtually ideal correction of chromatic aberrations when combined with optical glass. This fact has long been known, and in 1880 natural fluorite was already in practical use in the apochromatic objective lenses of microscopes. However, since natural fluorite exists only in small pieces, it cannot be used practically in photographic lenses. In answer to this problem, Canon in 1968 succeeded in establishing production technology for manufacturing large artificial crystals. Thus opening the door for fluorite use in photographic lenses.
Super UD lenses
The high cost of synthetic fluorite crystal production makes fluorite lenses extremely expensive. One answer was found in the latter half of the 1970s with the appearance of UD (ultra low dispersion) glass that could provide characteristics similar to fluorite but at a lower cost. While the indexes of refraction and dispersion of UD glass do not equal that of fluorite, they are significantly lower than those of other types of optical glass. Moreover, UD glass does display partial dispersion characteristics similar to fluorite. The selection of the proper lens element combination in consideration of the intended focal length and other factors can provide close to the same effect as fluorite, (two UD lens elements are equivalent to one fluorite element). Another breakthrough was made in 1993 when Super UD glass was introduced as a new material that achieves almost the same performance as fluorite while achieving a new balance of greater cost reduction and even higher quality.
