Scheelings
Well-known
The difficulty with focus shift in a lens like the 50/1.5 Sonnar is that if the lens is adjusted for accurate focus at f1.5, as you stop down the depth-of-field increases, but it shifts backwards sufficiently that the point you focus on is in front of the plane of acceptable sharpness - and therefore unsharp. If you focus on an eye you end up with a sharp ear. The increase in depth-of-field in the 50/1.5 Sonnar does not overcome the focus shift until ~f4.5 as this test shows quite clearly: http://photo.net/leica-rangefinders-forum/00aKSW
Focus shift is not imaginary, or an invention, but it's also not fatal. Understanding it can facilitate you being able to better use of lenses that display it, and make photos that look like you want them to, which is the point after all.
Marty
Ok, but if I focus on something at 1.5, take a shot, then step down to say 2.8, does refocussing correct it - or is this lens always out of focus - even if it appears in focus in the viewfinder - from F2~4?