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Originally Posted by crazee horse
er............. whats APO?
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''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.