[Leica] APO

[deveney.marty at saugov.sa.gov.au (Deveney, Marty (PIRSA))]

Mark is right:

>To me there are two definitions of APO.
>One for the better lens design companies and one for the unwashed masses.

Here:
http://leica-users.org/v13/msg13490.html
Erwin points out that:
"Now there is no industry norm that first describes 
which glasstypes you have to use to make a lens an "apo" and secondly 
describes which numerical deviations are required for such a 
designation."

If you use the absolute definition: i.e. an optical system corrected so that
it gives three images of identical size for three different spectral lines
or regions, you'll find that there are no APO lenses in current production,
even including those made by Leica.  There are, however, several lenses that
are lens that has been corrected to a greater degree than in most other
lenses for the three primary spectral colors, including the R&M 90 APO
ASPHs.  The differences in image size and therefore the 'degree' to which
the Leica APOs are APO is an interesting study in itself.  One thing is for
certain, they are better colour corrected (closer to the absolute APO) than
any other lenses manyfactured.

Anyone with sufficient masochistic tendencies can calculate the
(apo)chromatic error.

The other little-discussed fact is that the chromatic error decreases with
stopping down, at least until diffraction makes it worse.  At f8 or
thereabouts, it is minimal, in APO and other optical systems.

The true APO lenses for my Leica microscope cost about the same as a lens
for a Leica camera, but are about the size of your thumb.  It is very
difficult to achieve with camera lenses and not really necessary.  'Good'
correction, such as is offered by the Leica APOs (excellent, really) will
very, very rarely show any chromatic abberration.

Later,

Marty