This example shows the f/2.8 performance problem in the field. See also the Mosaic comparison.
The frame used here is the one used on the Hot Spot page. I shot this scene 4 times, with Live View focus and autofocus. Same results.
Show below is a crop. Click for an actual-pixels crop, mouse over to see the difference. (The f/4 frame is darker at center because of the hot spot problem at f/2.8).
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