More to be added as I gain experience with the 645D.
Initial testing suggests that the autofocus system is generally very good, but several suspicious frames suggest to me that focus errors do occur.
The 645D has a fine focus adjustment feature (custom function #33), but I found in further testing that inherent variability in focus precision makes this less useful than it might seem. I also found that depending on the direction of focus (near to far or far to near), the behavior varies in terms of the amount of error.
The image below shows a tree on which focus was made at dead center. Yet the focus is well into the trunk, as can be seen by sharp bark at the periphery (the tree is very large, perhaps 4-6 feet in diameter.
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