A few from a Great Heron (5DIII + Canon 135mm at f2, 1/2500 s)
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which brand of extension tubes did you use?
5D mk III
Should I return it to canon or the the dealer (B&H) since I received this lens with the 5D Mark III less than a month ago ?
I'd start by calling B&H to see if they'll just swap out the lens for you. Else, you'll need to contact Canon Service.
Looks like decentering to me, swap it out if possible, or send to Canon, but do it!