I followed this tutorial:
https://vimeo.com/6580446 [vimeo.com]
(Render extra image plane with Pz, 32 float, Closest Surface, minmax min. Then a Dof node combined with a defocus node set to “per pixel defocus”)

The effect works as it should, but only in areas with background geometry. In ares where there is nothing in the background, the foreground objects get blurred, but the blur does look weird. You can see this in my screenshot.

I experimented with the different values in the dof and defocus nodes, didn't work. The only thing that improved the image a little was to blur the mask outputted by the dof node, but this can't be a solution, right?

Help is greatly appreciated.