I have set the camera to have a depth of field within a certain area using the widget controls. Is there a way to strengthen the blur of objects outside this DOF, like a parameter in the camera or in mantra? Other than adjust the cones in the widget I mean.

The "fstop" on the Sampling tab of the camera controls the strength of the blur, just like a real camera.

Thanks, the F-Stop is the same as moving the cones on the widget. Not quite what I meant. I want to increase the blur without moving this, which acts like a region of focus. If you adjust F-Stop it also changes the region of focus.

litote
Thanks, the F-Stop is the same as moving the cones on the widget. Not quite what I meant. I want to increase the blur without moving this, which acts like a region of focus. If you adjust F-Stop it also changes the region of focus.

No, that's not possible. Changing the depth of field continuously changes the region in focus. In reality, the depth of focus is always 0. The cones are there to illustrate where the area likely to be perceived as in focus is located. F-stop is the only control for this.

Edit:

Performing depth of field blur in post using compositing software is an option that would allow for non-physical behavior that you describe.