Nikon ED - Manual (Page 13)

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ABOUT THE
NIKKOR
ED SERIES
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
dispersion (called the “secondary spectrum”) which limits image
contrast 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 exposures is un­
necessary. The remaining lenses such as this Nikkor ED 800mm
f/8 IF, 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 conditions.
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