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...What i am not sure, is if the MA works accurately always. ...I did the MA, and i found that the lens was back focusing. At -6 my photos were sharper indeed. The problem is that when i was shooting in differnet distances (much longer or too close) than the distance i made the MA, my results were less sharp again...
Hopefully even though the other distances weren't as sharp, they were sharper than with no MA at all.
If you have one distance range that you use more often than others, doing the MA at that distance is best. A birder might always take pictures of distant subjects, for example, while a portraitist might always take close-range shots.
For someone who uses the entire near/far spectrum, many people suggest doing the MA adjustment using a test target at a distance of 50-times the focal length. So for a 400mm lens, for example, you'd place the test target at 400 X 50 = 20,000mm = 20m = 65 feet.