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T2.2. 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, particu-
larly 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
frontand 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 300mmf/2 IF-ED, have been built
exceptionally compact due to the useof ED glass
elements; for these lenses, however; corrective
re-focusing 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.
