What does APO stand for?

[Harry Settle]

On Sigma lenses, I can't find a definition for the APO designation.

[Olegis]

As found here (http://www.vividlight.com/articles/413.htm) :
APO - Indicates the use of apochromatic lens elements. APO, like Nikon's ED glass and Canon's UD glass. These low dispersion glass elements effectively eliminate chromatic aberration or color fringing resulting in sharper images (chromatic aberration is particularly troublesome in fast telephotos).

[scottbergerphoto]

From Sigma's site:
http://www.sigmaphoto.com/html/lenses.htm
Scott

[Harry Settle]

That was really quick. Thanks