Final gather is very flexible and many different factors can contribute to its effect. You’ll need to experiment with settings to achieve the look you want.
If you are not adjusting the camera position, the position of any objects, or any Final Gather attributes, you can store and reuse Final Gather illumination results to speed up rendering.
To render with Final Gather
Select Windows > Rendering Editors > Render Settings or click the Render Settings window icon.
Final Gathering (in the Rendering Features section of the Features tab) is automatically enabled.
If the default settings are not sufficient to achieve the look you want (or to reduce render speed), tweak the Final Gather settings until you achieve the look you want. See Final Gathering for more information.
If your scene contains caustics, global illumination or final gather, and you would like to see those effect during mental ray IPR, in Render View select IPR > IPR > Render Settings.