out of focus effect

I have applied defocus node to obtain out of focus effect, but it has an artificial effect
http://img161.imageshack.us/img161/4001/artefpe5.th.jpg

here is the detailed image
http://img179.imageshack.us/img179/7020/artefdettaglio1km7.th.jpg

You can see that the first out of focus ball seems to be covered by second ball

here is my settings
http://img150.imageshack.us/img150/6727/screenshotkq4.th.jpg

how can I avoid the problem?

thanks

i think this is what the Z-mask is designed to correct, in the next release. see blender.org/development