Wide open, the 70-200mm f/2.8L IS II is so much better than the original model at f/2.8, that this factor alone might be reason alone to upgrade for those who regularly shoot at f/2.8 - f/4. Gone is the dull veiling haze of the original model, instead we see excellent detail and high contrast over most of the frame at f/2.8, with only the edges and corners showing slightly low contrast. Only a lens like the 200mm f/2L IS can improve upon this performance.
The 70-200/2.8L IS II appears to have performance in reserve for a future 30 - 40 megapixel camera, but that presumes perfect optical alignment of lens, camera and sensor, along with exacting focus precision.
Precision requires care
The highest level of performance seen with sample #6 takes a bit of luck: two of the first five samples were as sharp but skewed left/right, and three were clearly inferior.
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