First, attach the lens shader to the MayaCamera mental ray rollout, ‘Lens Shader’ slot.
Done that access the Rombo.Camera.
Generally the default parameters are already good for most situations. Take care that of course even in Pinhole mode (ie. no DOF) the Aperture is still relevant for tonemapping.
If you don’t plan to use a specific SensorResponse color function, than just leave GAMMA_0_4.
Use Cm2 factor if your scene is kind of un-balanced between for example the Environment shader ( for example if it’s not HDR) and the tonemapper. You may want to use plausible Aperture and ShutterSpeed but if the Env shader is not HDR it will render very dark. You can increase the Cm2 factor to let the Env render correctly without change your camera setup.
A simple scene with basic tonemapping.