It seems like a few months ago there was a discussion about a Nikon telephoto lens, and there was a link by an expert to a comparison he wrote in another site or mag comparing the Nikon lens to the (I carefully wrote this down) Sigma 70-200 f 2.8 EX DG OS HSM. After reading the review I decided that at approx. $1,000 less, the Sigma lens would be fine for me, pretty much an amateur. So recently I ordered the Sigma lens. I am an aspiring birder on the doorstep of retirement. But after ordering the lens, I see there is a Sigma lens with the same labels and features except the model or whatever is preceded by "APO". And I am unable to find out what that means. The lens I bought from the listing does not have these letters. I intend to use the lens on my Nikon D5000. Any information would be appreciated.