What are ED elements?

Question

I find that some lenses are indicated with the letters "ED", referring to some special type of lens element. What are ED elements, and what advantages do they bring?

Note that this question isn't very clear on ED elements.

Asked by DragonLord

Answer

ED means extra-low dispersion, referring to a type of glass that to disperses light less than ordinary glass. Dispersion means breaking up light into its constituent colors due to bending different wavelengths of light to different degrees. Because dispersion can cause chromatic aberration, ED glass elements can help reduce purple fringing and other chromatic aberrations. ED elements tends to be limited to higher-end lenses because ED glass tends to be more expensive to manufacture and work with than ordinary optical glass. Canon takes this further in some of its L lenses with fluorite, which has the lowest known level of dispersion, but fluorite elements are extremely expensive to manufacture (especially if very large--the Canon EF 1200mm lens demonstrates this) and incorporate into a lens, and tend to be fragile.

Answered by DragonLord

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