Now shift-select the camera and load this selection into the hypergraph. Now drag and drop the distance node over the reverse node and use other to open the connection editor. Use the distance attribute at the end of the distance shape node attribute list and Input / Input X on the right side (reverse node). Now drag and drop the reverse node over the lens shader and use again other to open the connection editor. use Output / Output X on the left side and Plane on the right side. Play with the Radius attribute to change the effect. One thing you should have in mind is, that the sampling values needs to be upped. That's the only bad thing on this. But you can set-up an post-blur effect with distance dependents in mental ray too...
