http://diglloyd.com/articles/LensAndCameraIssues/BrandNewBlur.html

There is no such thing as a free lunch;
… or a prefect lens, camera, car – or whatever.

As mechanical gear is produced, to keep cost down a consideration of importance is made, which then leads to a “tolerance” – how much each part can deviate from “the perfect measure” and still work the expected way.

The same goes for electronics, optics, clothes, whatever you can think of.
Of this comes the variations in performance; it is everywhere around you.

Sometimes the less than optimal measures add up “the wrong way” by coincidence.
Not nice to be the victim, but it happens – for one thing; “warranty” is meant to allow you to remedy this.

http://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths

http://www.slrgear.com has a couple of interesting articles:
February 14, Multi-sample review: Nikon 50mm f/1.4G AF-S
January 31, Lens sample variation – A closer look

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