VrayPhysicalCam and depth of field indication

[Azaik]

For the life of me I can't figure out how to set the depth of field for a VrayPhysicalCam. I understand how DOF works, how it changes with aperture and so on.

 

My problem is that the planes shown for a VrayPhysicalCam in the viewport (the ones that look like a standard camera's clipping planes) don't seem to match up with the DOF at all. There are three planes shown, and I assumed the middle one was the focus point, with the outer two indicating DOF - in other words, the area in which focus begins to drop off and become blurred. You could then see at a glance which part of the image would be in focus.

 

But that doesn't seem to be the case. Anyone know what these planes are meant to indicate? And is there a simple way to get a visual indication of DOF using them? I've tried fiddling with targeted/free cams, the focus distance and target distance in various combinations, and none of it seems to work as I'd expect. The Vray help files online don't mention these planes at all.