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?