Create a new render layer and add all your objects to it. Open the attributes on the render layer (RMB > Attributes). Set the render layer preset to Luminance Depth. Follow the connections to setRange node. Break the connections on the Old Min and Old Max. Set values for these such that when you render, the foreground is nice and bright and the far objects are close to black (these values are distance from camera, which you can determine using the Object Details in the HUD). Hope that helps.