Central sharpness is just fair at f/2.8, but very good by f/4 and excellent ... Vignetting is not altogether unwelcome with a 14mm lens; in fact, a modest amount can be quite beneficial in ...

Central sharpness is very good at f/2.8 and f/4, excellent at f/5.6 and f/8, very good at f/11. It is still good at f/16 and f/22, but diffraction has taken hold and takes the edge off the crispness.