I hope this link explains http://en.wikipedia.org/wiki/Apochromat

Don

''Normal'' lenses are only designed to accurately focus one wavelength of light so on longer focal length lenses you can sometimes see the other colours around the edge of the focussed image (not the same as the purple fringing you get from the properties of the digital sensor but looks rather similar). More expensive telephotos or tele-zooms correct for this and are called apochromatic lenses. Sigma call their versions of this APO lenses. They produce sharper images but are more expensive. The effect does not show up on shorter focal length lenses under say 300mm. Hope this helps.

By the time I had finished typing my message and posting it Don puts a much better answer up! Ah well ;) .

I have used this lens and the APO version, if you can afford it the APO is a significantly better lens. The price of the one in the link is about what you should pay for it in a shop so have a look around before buying, the APO woud be ~£50 more.

bugger, ive just bought the 70-300 hope im not dissapointed.....................

Let us know. I think it will be fine.

APO = Apochromatic
It basically means one or more glass elements within the lens is of a higher quality than a non APO lens. Sigma terminogy...other Sigma lens options (off the top of my head) the 70-300 offer both APO and non APO versions...about £50 difference in price. Well worth it if you're after the sharpest lenses. Nikon call the same sort of thing 'ED' (extra low dispersion glass), Tamron 'LD' (low dispersion)...etc ....
Pick up a free Jessops catalogue at your local shop, as there's some lens reference guides in there which explain those letters which make appearances on many lenses now.

I can't offer anything additional with regard to the above lens, but wife Jacky has a 100-300mm f/5.6 Canon lens, despite being quite noisy focusing mechanism, it is quick, and is quite a nice sharp one, particularly at the 300 end.

Ooops, sorry...didn't spot there was a second page to this thread!:o :rolleyes:

well for the last week ive watched the front door like a hawk, ready to pounce and snatch my lens out of the posties hands and snap away like a lunatic, i couldnt believe how long the lens is taking so i checked my inbox, and when i ordered it apparently i never specified canon fitment or a contact number so it hasnt even been dispatched yet! AAAAAAAAAAAAAAAAAAAAAAAGGGGGHHHHH!

Well that gives you a chance to change your order to the APO version if you want to! :)