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.
Select Window > 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 Gatheringof the Rendering guide for more information.