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Matt there are some theories that say that introducing a moving lens element compromises sharpness. Whether that assumption is based in accurate observation or is just speculation is debatable. I've also seen written that the lens construction is improved with the IS optics to ensure it is as sharp as the non IS version. I suspect that if you tested several side by side you wouldn't see any difference. One thing that is consistently reported is that the f2.8 is more prone to flare than the F4 and that the F4 is sharper.
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