Hi there,
Run this MEL script to control the samples for Maya’s built in camera DOF effect:
addAttr -ln "dofLensSamples" -at "short" mentalrayGlobals
After that, find the mentalrayGlobals node (Outliner->turn off DAG objects only).
At Extra Attributes you can find a new attribute called Dof Lens Samples.
Cheers, D