APO or not

[Busted]

Hi All
I'm looking for a zoom lens from 70-300mm or 100-300mm and currently looking at the Sigma 70-300mm f/4-5.6 APO DG MACRO or the Sigma 70-300mm f/4-5.6 DG MACRO. Could anyone please tell me what the APO stands for? It will be used on a Pentax K-X.
Also like the sound of the new Pentax 55-300mm but i cant quite afford it. Anything else i should be looking at?
Cheers Frank

[jjphoto]

Apochromat explained.

Essentially, APO lenses should have less chromatic aberration (all visible colours, and often infra red, will focus at the same point instead of focusing in slightly different places due to the way different colours are affected by a lens) than non APO lenses. In simple terms this makes a sharper image with less colour fringing at the point where the image is focused. There are no rules about the use of the term 'APO' so some times I do wonder how APO some lenses really are but I would choose an APO over a non APO lens any day of the week. I have a few APO lenses, Leica APO Telyt-R 180/3.4, Leica APO Summicron-R 180/2.0, Rodenstock APO-Rodagon N 80/4 (enlarging lens, but I use it for close up/macro), all of which perform exceedingly well. When a reputable manufacturer calls a lens APO then it will generally be of a very high optical standard.

JJ