Focus shift is detail is found in Making Sharp Images.
Focus shift is a destroyer of image sharpness in the sense that it dulls the micro contrast where the photographer intends it to “land”; the zone of peak sharpness is shifted in its entirety, generally to the distance.
John V writes:
Reading Making Sharp Images is certainly proving valuable for this lens.
I am also seeing substantial focus shift, perhaps ~9mm at f4 in close range. Ridiculous. I also tested at long range, 15m or so, and there is more than enough shift to trash the sharpness. Using this lens anywhere but wide open requires keeping a finger permanently on the DOF preview button. Yikes.
A shame too, since it feels great in the hand, it's a fun focal length, and the color looks punchy. I really wanted to like this lens. But it's getting returned, and your similar results makes me confident mine isn't just a bad sample.
I'm also seeing forward field curvature, for what that's worth.
Live View of the Olympus E-M1 could in theory offer a consolation by allowing manual focus while stopped down (by pressing the DoF button), but the 14.4X magnification is so smeared that it is an exercise in frustration and eye strain. Plus the depth of field grows quickly with stopping down; it’s just very, very difficult to deal with.