I checked "Separate Process" in the render tab of general preferences, and selected two threads.

I made a simple scene with two props, one far behind the other. I checked Depth of field and Smooth polygons in the render settings (Quality : Auto Settings : Final). I calculated with a Python script the distance to the barrel and inserted that distance (in feet) into the focus_distance dial of the main camera.

After pressing the render button I received a message about my Firewall and ffrender.exe, choosing to let it pass through.

The render worked this time, apparently when using just one process earlier, the Firewall message was behind something else and was invisible, blocking the whole process. Weird that a normal firefly render didn't have the same effect ! Anyway here is the result and I must admit I don't like the blur that much, any way to make it render better ?

Thanks again for your help

Lunedust