This lens is infamous for corners softness and color fringing.
It turns out F/4 is not great either on M43, but F/5.6 i much better, and F/6.3 is totally usable: the corners softness is gone, and color fringing is easily corrected in Lightroom.
It makes a very unbalanced setup, but as an alternative to buying an Olympus lens, it does the job. Manual focusing is fats and smooth, and the equivalent 35mm focal length of 24-48 pretty usefull.