Just published in Making Sharp Images is an extensive study of focus shift with the Canon 50mm f/1.2L on the Canon 5D Mark III.
This study is also cross-posted in DAP: study of focus shift with the Canon 50mm f/1.2L on the Canon 5D Mark III (in DAP).
Canon users shooting the Canon EF 50mm f/1.2L will find this study immensely instructive. The most common complaint about the 50/1.2L is that it is “soft”. This is absolutely incorrect (it is very sharp), but the culprit is a very strong focus shift. However, the “soft” statement is absolutely true if one fails to compensate for the focus shift.
For many lenses (especially f/1.2, f/1.4, f/1.8 lenses), by far the most important lens parameter determining lens sharpness is focus shift. Which is why many controlled tests are completely ignoring the most important performance issue! A sort of 1.735 children thing, or being half pregnant. Know your gear.
Focus shift and what to do about it are discussed in detail in Making Sharp Images.
Nikon and other brand photographers should take the time to read this study, as there are plenty of Nikon (and Zeiss and Leica) lenses with focus shift.
Toggle the image below to get a sense of the shift. Far larger crops are provided in the study, with full aperture series from f/1.2 - f/11.