It still looks a bit dark in the corners at f/5.6... or you have something in front that vignettes, like a hood.
My feel of the ZF.2 18 is that it's a good lens around center at all apertures, but vignettes strongly wide open. The vignetting clears up as it's stopped down and as far as I can tell is gone at f/6.7. (I use half stops.) Corners and extreme edges (like the last 5%) never get super crisp, but are fine - just slightly soft even at the lens' best. It's hard to tell what they are like until stopped down a bit though since they're dark wide open. Beyond f/6.7 I see no improvement and all that changes up to f/13 is DoF. At f/16 there's an ever so slight loss of contrast due to diffraction. There is some chromatic aberration at all apertures, which is easily removed in post. Color is excellent, making for lovely results if you shoot in the right light. It can take a Lee filter holder (using an 82mm lens ring), but not a shade as well. It can use a wide Lee shade w/o a filter holder. (I don't have a WA shade with a slot; something I really need to correct at some point.)
Basically, for me and my uses it's a keeper...
It's nowhere near as good as the 21.
