Hi Frantisek,
Here is my translation of Sigma's explanation from their
Japanese web site;
Sigma calls our lenses with minimum chromatic aberration
"APO" series. <snip> With normal glass, low dispersion
concave lenses and high dispersion convex lenses are used
to correct chromatic aberration. However, this way, you
can only correct chromatic aberration of, at most, two
colours. The rest are called "second spectra". In APO
series, Sigma adopts ELD (Extraordinary Low Dispersion) and
SLD (Special Low Dispersion) glass in order to control the
second spectra to the limit. This gives our APO series
high contrast and excellent resolution.
I'm not so confident in the technical terms... Anyway,
from what they insist, Sigma's APO seem to correct three
colours indeed.
I haven't tested it in such a strict way, though.
Hope this helps you.
Sincerely,
****************************
Take Ueda, Osaka, Japan
[EMAIL PROTECTED]
http://members.tripod.co.jp/hayatama/photo/
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