This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.
Low dispersion glass is specially formulated to have a small variation of index of refraction with wavelength. This makes it easier for the designer to reduce chromatic aberration and lateral color. This kind of glass is most often used in long lenses. Marketing designators such as ED and SLD hint at the use of low-dispersion glass.
 
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