Achromatic, Apochromatic and
Super Achromatic LensesPhotographic lenses are classified according to
the type and quality of the color correction
features they possess. Achromatic color
correction techniques focus just two colors
(usually red and blue) in the same plane and
have been used since the 19th century. The first
photographic lenses were achromatically
corrected.
The color errors that achromatic lenses
nevertheless produce, known as ‘secondary
spectrum’, are very difficult to eliminate
completely. However, the effects of
secondary spectrum on image quality are
generally negligible if you use a good quality
achromatic lens.
Optical engineers are constantly searching
for new ways to reduce or eliminate
secondary spectrum and thus improve the
quality of photographic images.
Apochromatic lenses can correct three
colors. While achromatic lenses require two
elements made of different materials, an
apochromatic lens requires at least three
different types of element. Apochromatic
correction is designed to reduce or, ideally,
eliminate secondary spectrum by focusing all
red, green and blue rays to the same point on
the focal plane. The only way to do this using
currently available materials is if you areprepared to accept other types of aberrations
as part of the deal. Most lenses therefore
represent a compromise between faithful color
reproduction and a number of other optical
characteristics and anomalies.
Lens manufacturers build lenses to a
standard that they themselves are happy to
call apochromatic. However, it is probably
safe to say that Sigma’s definition of
‘apochromatic’ for a US$300 lens will differ
from the Leitz definition of the term in a lens
that costs 20 times as much.
The term ‘apochromatic’ is neither
standardized nor patented, so you can only
really find out what various manufacturers
mean by it by conducting a thorough lens
test. The incidence of chromatic aberrations
varies enormously throughout the zoom
range, particularly in budget telephoto
zooms that nevertheless carry the ‘APO’
label. Today’s market offers APO models in
the entire range from budget to high-end,
while some manufacturers whose lenses
actually earn the description don’t use it
at all.
At the very top end of the quality (and
price) scale are ‘super achromatic’ lenses that
are capable of correcting four different colors.
The Carl Zeiss Tele-Superachromat T* 5.6/350
(for Hasselblad cameras) is one example of an
extreme high-end lens with unbeatable color
correction.Even if it is not as commonly used as
‘apochromatic’, the term ‘super achromatic’ is
not precisely defined and is still used
according to the needs of marketing people
rather than photographers. The first super
achromatic lenses were made by Zeiss in the
1960s and the ‘Superachromat’ label has
remained a Zeiss trademark ever since. Zeiss’
direct competitor Leitz describes its equivalent
lenses as having APO characteristics, although
the products themselves are directly
comparable. Once again, even though the
description is based on definable technical
attributes, the presence or lack thereof doesn’t
accurately describe the quality or performance
of a lens.Close-up Lenses
and Teleconverters
If you interfere with a lens manufacturer’s
carefully designed system of elements and
gears, you run the risk of producing not only
optical anomalies but also additional chromatic
aberrations.
Using a teleconverter is a simple way to
increase the focal length of a lens, but is
almost guaranteed to increase chromatic
aberrations too. Teleconverters are normally
constructed with quite a lot of internal space
and some manufacturers use this to add
additional corrective elements.Chromatic Aberrations | Lenses
Achromatic correction – a standard feature in most
photographic lenses these days – corrects two colors
but leaves the third uncorrectedThe conventional
element (top) and
the DO element
(center) refract
incoming light in
opposite directions.
Used together, the
two elements
cancel out each
other’s aberrations.