Final gathering is an optional, additional step used to improve global illumination.
Final gathering increases the number of rays used to calculate global illumination (GI) to smooth out and eliminate adverse lighting artifacts. Because global illumination is calculated from a photon map, rendering artifacts such as dark corners and low-frequency variations in the lighting may occur.
By activating final gathering, you increase the number of rays used to calculate global illumination, which reduces or eliminates these artifacts.
Final gathering can greatly increase rendering time. It is most useful for scenes with overall diffuse lighting.
Tip: Leave final gathering off to preview the scene, then turn it on for the finished rendering. (Increasing the number of photons used to calculate global illumination can also improve global illumination.)
You activate and adjust final gathering in the Advanced Render Settings palette.