As an alternative to DOF. Place a box (or 2d plane, but I don't trust anything that doesn't exist sideways) behind the head. Scale it up until it covers the background, then smash it flat again. (The box should face the camera, I forgot to mention that.) Go into the material lab and set all the colors to white and all the number to 0. Make it 100% transparent. We're going to use Blurry Transmissions here, so adjust the specular highlight to something. (The whiter, the blurrier, might want to start at 127RGB and adjust from there.) Under render settings turn on Blurry Transmissons, set the RPP low for some test render. Test render. Fiddle with the specular highlight until happy. (Might also try fiddling with the depth of the box, I don't know if it matters, but it'd be fun.)