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You should move it to -.
What i am not sure, is if the MA works accurately always.
I had a 50D and a 400 f/5,6L in the past. I coulnd't get very sharp results even under ideal conditions. My friend with also a 50D and a 100-400 and his results was like mine..I was sure that the fixed 400 should get sharper results. 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..
I am not sure if it was my camera's problem, or the lens', but i noticed that MA was not working the same in all distances...
I may be wrong, i don't know for sure, but an other friend also told me the same once, and i wonder if anyone else had the same experience...
Bob, sorry about asking in your post, if you mind i could write it on a separate topic..