Coming back to the performance with close-up filter:
Did some more in depth testing with the Canon 500D
. And although the test-target is not optimal for 1:2.4 magnification you can still see what's going on.
After testing at 200mm from f/2.8-f/11 I found that flatness of field was quite amazing but at f/2.8 I never got satisfying sharpness - even in the center. Only after stopping down to f/8 were the results "good enough".
The surprise came, when I reverse-mounted
the close-up lens (a suggestion by Bjørn Rørslett
It was immediately clear from the f/2.8 live-view image that this was the sharper solution. The center-sharpness at f/2.8-reverse was as sharp as f/8-normal
See this 100% crop of the center
shot at 200mm f/2.8 plus Canon 500D mounted in reverse orientation (left) and normally (right):
All focusing was done manually with Camera Control pro in live-view. Contrast based AF would have chosen a slightly more contrasty focus-position in normal mount but sharpness was reduced then.
But if you look at the corners field-curvature
clearly showed now.
See this 100% cop of the DX-corner
at 200mm f/2.8 plus Canon 500D mounted in reverse orientation (left) and normally (right):
The corners look better with the close-up lens normally mounted.
Stoping down you could still see differences between normal and reverse mount even at f/8.0 and only at f/16 both images were indiscernible. But that is on a DX-body. Not sure how the corners would look on a D3x.
So if you're shooting stamps you should use the Canon 500D in normal orientation and stop down to f/8.0 at least. But when shooting three-dimensional objects you should reverse-mount the 500D which gives you amazingly sharp shots even at f/2.8.