UD Lenses
Although fluorite has superior optical properties, its high production costs make it unsuitable for widespread use. To extend ideal performance to a wider range of camera lenses, Canon set out to create a new type of optical glass with fluorite-like properties. In the early 1970s, Canon was successful in developing Ultra-low Dispersion (UD) glass. When compared to standard optical glass, UD glass has a lower refractive index, low dispersion, and excellent partial dispersion characteristics. As a result, an optimal combination of UD lenses can produce almost the same effect as fluorite. The development and implementation of these high-performance fluorite and UD lens elements resulted in the establishment of the "L-series" line of "Luxury" high-quality FD-mount lenses in 1978.
Canon developed Super UD glass in 1993, which outperformed conventional UD glass. A single Super UD lens can replace two conventional UD lenses, providing nearly identical optical characteristics to fluorite. Super UD glass, which was first used in the EF400mm f/5.6L USM lens, significantly reduces chromatic aberration while also contributing to a smaller overall lens size for improved portability. Super UD glass's exceptional performance makes it a key component of many L-series lenses.