blured backgound

I need in a still scene a heavy blured background but the rest of scene to remain sharp. So the classical DOF with a Defocus node don’t work in this case. So only two “areas”, one with no blur and one (the background) with big blur.

It’s possible to obtain that with only one render? (not to render the background, blur it in Gimp then import the image and render again)

An idea: it’s possible to use separate DoF nodes for foreground and background?

Divide the background and foreground objects into separate Render Layers (http://wiki.blender.org/index.php/Manual/Render_Layers) and then use a regular Blur node to blur the background and an alpha over or Mix node to place the Foreground over the blurred background.

It works with an alpha over node! Thank you very much!