[shuttertalk]

Here's an excellent article published by dpreview, authored by Roger Cicala from Lensrentals, a company which rents out hundreds of lenses each day and is arguably in an excellent position to comment on variation in lenses and the search for the "perfect copy".

http://www.dpreview.com/articles/7333489...-fallacies

If you think about it, every manufactured product has variations and minute differences between each copy, especially when lenses and cameras have many tens or hundreds of internal components. Each manufacturer has tolerances as to how acceptable these variations are.

The crux of the matter is though that:

But even the cameras and lenses that meet specifications are still going to vary slightly. Many people think, 'I’ll try eight lenses and take the best one.' The reality though is that the only sensible definition of 'best' is 'best with the camera body you are using,'. This is because there is plenty of room for variance in the behavior of bodies, too.

By all means, test all your lenses and make sure you get a copy that produces excellent images for you, but if 4 out of 5 copies turn out bad, then it just might be that your camera body might be the one with the problem. Also, he suggests micro adjusting your lenses if your body accommodates that function - it can make a big difference.