The longer the focal length of a lens, the more difficult it is to correct the chromatic aberration that causes colour fringing. Nikon developed ED (Extra-low Dispersion) glass to enable the production of lenses that offer superior sharpness and color correction by minimizing chromatic aberration.
The red/green chromatic aberration around the dove's outline (left) is reduced with ED glass (right)
Nikon developed several types of ED glass suitable for various lenses. They deliver stunning sharpness and contrast even at their largest apertures. The images below show how ED glass reduces color dispersion that occurs when light rays of varying wavelengths pass through optical glass.
Normal glass with secondary spectrum shows the effects of chromatic aberration
ED glass produces a smaller secondary spectrum, so less chromatic aberration
Super ED glass produces even less secondary spectrum and less chromatic aberration