Steve wrote:
: Kaysersoze wrote in message <01bce296$7a106d80$8c1c8118@strangel>...

: >So all Nikkor lenses are not good because they're not APO????

: It is funny that all the indies make a big deal about their lenses being
: APO, and mark them appropriately. As far as Canon and Nikon go (and
: probably Pentax and Minolta as well), pretty much all of their lenses are
: APO, they just aren't marked.


The other side of that coin is that APO has become
a relative term. It used to mean that three colors
[RGB] were brought to a common focus *AND* at equal
magnification. lately, thanks to marketing efforts
by Sigma and others, it may mean only that the lens
has significant improvement toward being apocromatic
when compared to a "similar" achromatic lens [two of
three colors at reasonably close to same focus with
the third [red] focused reasonably close to the two.

Achromats are the most common design, which is why
published tests may say such things as "red fringe
is very minor...". Red defocus was unimportant when
most films were totally or partially red-blind, so
it was blue and green that were traditionally made
to match focus.

True APO lenses were originally not for pictorial
photography but were used on graphic arts process
cameras to shoot halftone color seperations. The
dot pattern hides any real sharpness, and the size
of dots is governed by exposure, not optics, while
the sharpness of the dots is governed by processing
so APOs were not required to be extra sharp as long
as all colors focused to the same plane and at the
same magnification so that the four color seperated
halftone images properly overlapped [registration].
Such lenses shot flat copy onto flat film, so flat
field was also required. Today's pictorial APO may
may have curvature of field even though all three
colors reach a reasonably common focus everywhere
within that curved field. Before it was so abused,
the term "APO" was shorthand for "flatfield APO"
because in the major, or only, application of APO
lenses, flat field required of lenses whether they
were apocramats or achromats. Flatfield achromats
served well for making monochromatic halftones and
duotones on red-blind litho film. Color seperation
halftones require panchromatic lith film.

These APO flatfield process lenses found their way
onto view cameras as the "ultimate" quality lenses
since in achieving flatfiels apochomaticism, nearly
all other aberations were minimized. Even these
"ultimate" optics would never have found universal
application even *if* they had been less expensive
than other lense [not the case], because they were
usually slower than f:8.0 and had narrower field of
coverage than typical lenses. Yet, where speed and
angualr coverage were unimportant, APOs were the
ultimate in image quality, thus the term "APO" came
to mean "ultimate quality" rather than a specific
set of parameters, at least in general photography.
Once that meaning came into play, it's easy to see
why APO became an abused term.

David Rosen golem@capital.net golem@acmenet.net

A.R. van Ekeren wrote:
: Is it true that the APO correction is only important in tele-zooms ?
: Does it mean they have aspherical elements ?

True. The problems that APO design minimizes,
chromatic aberation [lateral and longitudinal]
increase directly with focal length, and are
also exacerbated by increases in lens speed,
so that there is little difference at smaller
stops or with slow designs between apochromatic
an achromatic versions of similar lenses.

David Rosen golem@acmenet.net golem@capital.net