[Klaus]

I have been using this little gem now since April '15 and I cannot confirm the heavy vignetting, in fact I am nothing but amazed by the performance of the lens. The price/performance relation is excellent.

There's more where this comes from:

 

That's hardly a shot made at 11mm f/4 ... :-)

 

And it all depends on how you process the images. The vignetting figures represent the original output.