Nikon NIKKOR ED 400mm f-2.8 IF - Manual (Page 17)

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 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.
17