My bitter experience with decentered lenses (lastly this Tamron and then the Nikon 24-120/4.0G VR) taught me one thing:
Decentering is easy to overlook in everyday shooting situations because you have to be very clear/exact where your plane of sharpest focus should be in an image. This was the reason I didn't immediately notice the decentering in those two lenses because I was happily shooting street scenes and nature-pics.
The best bet to see decentering clearly is with a shot from a considerable distance wide open (naturally) and mostly at the long end of a zoom. And with "considerable distance I mean a distance of >1000 x focal-length. So with a 24-70mm lens get away some 70-100m from (a) either a straight row of buildings or (b) a straight row of trees a long a street. Try to shoot pretty perpendicular to the row although at that distance it is not too critical. Keep the camera tilted to the left so that the row goes from one corner to the opposite corner in the viewfinder, focus on the subject (house, tree) in the middle and shoot (with a short enough shutter to avoid shake at all cost, or even better from a tripod). Now tilt the camera to the right so that the row goes through the other two corners and shoot again. If the difference
in performance in the left corner and the right corner bothers you: send the lens back.
(Note: you may keep the camera level and just do one shot to compare the left border against the right border, but the dual-shot-tilted method is safer).
Have a look at the following image in full resolution:Tamron 24-70f2.8 VC @70f2.8 54685
, on Flickr
and compare the resolution in the foliage on the left side against the right side. This is what I'd call a clearly visible difference.
There's only one caveat: this image was captured with a 36MP FF/FX-sensor - f you use a 18MP sensor differences might not be so clear.
Main message of this lengthy post: shoot, shoot, shoot. And if you're not satisfied, return the lens. No-one else can make the decision for you.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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