DOF Falloff Adjustments?

Is there a way to adjust the falloff of the dof for the vray camera? I understand the f-number effects this defocus area, but it seems to affect the focal area primarily, not necessarily the fallout curve. There are times when I would like to set the defocus spot, but then adjust falloff curve a bit, by tightening or loosening the falloff of the defocused spot…perhaps based on a bezier curve or something.

Anyway to do this, or am I not understanding the f-number entirely. TIA.