I got a request for the example scene I posted pics of above, also my prior link is broken since I've changed hosts since then. Here's the link for the example scene. To change the near/far limits just move the bigger planes. To change the falloff you have to select the camera shaped controller and change it's Falloff…
There is a filter for applying DOF in post from a pass like this in After Effects which I have used in the past and works pretty well. I don't see why you want it to go from black in the foreground through white in the center and back to black in the distance though... The filter should deal with this. Normally you just…