ED (Extra-low Dispersion glass) Glass
Nikon started using this only in their super speed super teles in the late 1960s. These lenses say "ED" on and have a gold band around the barrel.

ED glass is an improvement over the fluorite used by other makers at the time because it is hard enough to use for outside elements, unlike the soft fluorite. Nikon use this for its expesive lenses.

ED glass helps eliminate secondary chromatic aberration (green-magenta color fringes) which is what previously prevented the design of practical super speed, super sharp super telephoto lenses.

ED glass is less stable with temperature than conventional glass, and so the focal lengths of these lenses change slightly with temperature. Therefore there is no hard infinity focus stop on ED lenses because the point of infinity focus will change a bit with extremes of temperature.

ED glass also has a lower index of refraction so it requires more deeply curved elements for the same focal length.

Nikon especially developed ED (Extra-low Dispersion) glass to provide pricise optical colour correction.
Because colours of the spectrum have differing wavelengths, red and blue light rays are bent at slightly different angles as they pass through nomal optical glass.
This dispersion results in colour fringing and lack of sharpness (see illustration at right).
Modern techniques to correct this chromatic aberration are effective with most lenses.
But telescopic lenses magnify the slight variation in focus between red and blue light rays.
Nikon developed ED glass in order to provide the sharp, clear resolution required for superb photographs

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Normal glass

Focus shifts with colour due to differences in
refractive index, resulting in color fringing.

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Extra-low dispersion glass

Variation in refractive index is small, so 
focus is less affected by colour and 
chromatic aberration (colour fringing) is reduced 
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