The center is already very sharp at the full opening of F/4; maybe there is a very slight improvement going to F/5.6 or maybe not. The far corners, however, do suffer from some fuzziness (see branches at top-left), disappearing at F/5.6. This will not stop me from using this lens at full aperture.

At F/4, the lens shows vignetting in far corners; at F/5.6 it goes away, although full images still show some. I'm not worried about it: it took me a month to notice it at all.

The camera has an option named Shading Compensation, which takes care of the problem during the raw-to-RGB conversion. So far I had this turned off; maybe it is time to reconsider.

From F/4 to F/11, center and off, an exemplary performance, peaking somewhere near F/8.0.

No vignetting, even diffraction only starts stirring at F/11. Just beautiful.

While the differences over the range of F/4 to F/11 are really minor, the apertures of F/4 and F/8.0 seem to be better than the others.

Again, an impressive performance at this focal length.

For most wide-to-tele zooms the long end is where they do worst. Not this one. Again: best performance at F/5.6 and F/8.0, quite OK (read: "better than I will ever need"), even at F/11.

I'm having a problem with the F/16 sample: it is more blurry than the one at F/22 (not to mention F/11, as it should). My shooting procedure rules out focusing error, so I'm suspecting camera shake (even if it doesn't look like that). The third option, the camer switching the sharpening to a more aggressive setting beetween F/16 and F/22, although theoretically possible, seems very unlikely. Anyway, I have to disregard this sample; sorry.