Hi,
I'm using LW; if I'm not mis-understanding your question, you don't have to use nulls or anything. In your Camera settings, crank your Antialiasing to "Classic Enhanced Extreme". I chose this for a reason; if you have "FPrime" from Worley you can see DOF very easily (lower settings work, too, this is just my preference). Then under Stereo and DOF, check DOF. In one of the "normal" views (top or right) you will see a black dotted line circle around the camera. Move the "Focal Distance" spinner until the edge of the circle lines up with what you want to be in focus. That's it! LW defaults to Lens F-stop of 4.0. Change this to 8.0, it's a better starting point. In FPrime you can now see DOF in real-time (sort of). If you don't have FPrime, do a "Shift F11" of the scene. Give it a second; in your real-time preview window you will see your DOF settings.
In LW we don't need to think about this stuff so much, it's done automatically. In Maxwell, you have to treat the camea as a "real" one, and set the settings yourself. Frankly, I like this better (I like to know what's going on instead of having things done for me). Hope this helps.
Mike Tripoli