Focus shift in fast 50mm lenses

Just published in DAP (see description): Focus Shift in 50mm Lenses — or why your fast lens is sharper than you might think, just not where you expect it. This is a prelude to a forthcoming review of the Sigma 50mm f/1.4 EX DG.

Focus was on the eye at right, but it is blurred—why?

Focus shift is caused by spherical aberration and affects most fast lenses out there (f/1.2 and f/1.4 lenses). Understanding when and why this happens, and what to do about it will raise your keeper rate, and (maybe) keep you from sending in your camera for service for non-existent autofocus errors.