The rule of thumb is that if you truly have a "bad" copy, you will know."First, the practical conclusion. Test every new lens you buy, right out of the box, and make sure itís reasonable. Speaking as someone who has lots of test charts and moderately fancy equipment to do that quickly and efficiently, Iíll just say you donít need it. Just find a couple of brick walls, fences, etc. that are in a flat plane and take some pictures. A bad problem will be evident very quickly. If the lens is bad, send it back to the store for exchange, not in to the manufacturer for repair (Because certain manufacturers are very likely to say ďimpact damageĒ and deny warranty even if you just unpacked it. No, I wonít name names, I donít have strong enough evidence to say one does this more than another.). Which brings up another point: donít buy anything from a place that wonít take it back. But donít confuse the need for autofocus micro-adjustment with a bad lens, itís not."
(from lensrentals.com, http://www.lensrentals.com/blog/2010/03/this-lens-is-soft-and-other-facts
That said, there is sample variation among lenses. The folks at Lensrentals.com (who have a large number of samples to play with) have written about this quite a bit.http://www.lensrentals.com/blog/2011/10/notes-on-lens-and-camera-variationhttp://www.lensrentals.com/blog/2011/10/the-limits-of-variation