Nikon NIKKOR ED 1200mm f-11 IF - Manual (Page 13)

Accessories
Slip-on leather front lens cap 122mm screw-in filters
Rear lens cap LF-1 Teleconverter TC-200
Built-in 39mm screw-in filter L37C Teleconverter TC-300
Gelatine slip-in filter holder Teleconverter TC-14
Other 39mm screw-in filters Aluminum Trunk CT-1203
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, such as this Nikkor ED 1200mm f/11
IF, have been so fully corrected that image sharpness extends
uniformly to the infrared region; for these lenses, corrective re­
focusing for infrared exposures is unnecessary. The remaining
lenses have been built exceptionally compact due to the use of
ED glass elements; for these lenses, however, corrective refocus­
ing 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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