Valera Fooksman » from archive
LensRentals.com - "This lens is soft" and other myths - http://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-...
Simple and valuable explanation of focus problems with shallow-depth lenses (f2 and below I think): combination of calibration tolerances of the body and the lens (both properly calibrated) can produce out-of-spec result. High-end bodies have a tune-up feature for this. ‎- Valera Fooksman