Correction of chromatic aberration in camera lenses has been limited, for the most part, to the use of techniques which bring two wavelengths of light, normally blue and red, to a common focus. Although known as "achromatic," lenses employing these designs exhibit a certain amount of undesirable residual disper- sion (called the "secondary spectrum") which limits image con- trast and sharpness, particularly at full aperture. Telephoto lenses are most prone to the ill effects of chromatic aberration, since secondary spectrum increases with focal length.
The Nikkor ED series consists of telephoto and super-telephoto lenses fitted with elements made of a Nippon Kogaku-developed special optical glass called "Extra-low Dispersion" (ED) glass. Although the optical characteristics of this new glass are similar to those of calcium-fluorite crystal, ED glass possesses a more constant refractive index over a wide range of temperatures and will, therefore, cause less of a focus shift. Also, ED glass is much harder and more resistant to scratches, enabling its use for front and rear lens elements to obtain optimum correction of chro- matic aberration over the widest possible wavelength range. Some lenses within the ED series have been so fully corrected that image sharpness extends uniformly to the infrared region; for these lenses, corrective refocusing for infrared exposure is unnecessary. The remaining lenses, such as this Nikkor 400mm f/2.8 IF-ED, have been built exceptionally compact due to the use of ED glass elements; for these lenses, however, corrective refocusing for infrared photography is necessary. Regardless of which type, ED-series lenses offer exceptional sharpness and full contrast for the most precise photography under the widest con- ditions.
