simply put, if you want less grainy effects (gather, DOF, ray-trace sharp shadows) you need to increase the pixel samples Increasing pixel samples increases the quality of effects that happen after the pixels are being shaded, like DOF or 3D motion blur. The quality of the shading itself is affected by the shading rate - lowering that will make gather and raytraced shadows render more precisely (and increase render time). What can also help reducing the grainyness of images is changing the post filter to a 2 pixel gauss filter instead of the default 1 pixel box.