I try to render light, coming through a small opening and the result looks horrible and noisy where the light hits the volume. It doesn´t matter if I create the volume with a large cube and assign a shader or add it in the world settings. The renderresult is always the same. I used a denoiser and pretty high samples and it is not even close to acceptable. On top of the ugly result, the rendertime goes up and seems to change nothing.
If you are using a small window opening you might want to consider using the portal feature on your area light if you haven’t done so already. Likely not enough light is making it in. See below for more details:
The branched path tracing realy caused some problems. I dont know the reason, but after using path tracing it already looks better. The other problem was caused of some details on the wall behind the light cone. The denoiser turned the details together with the fading light into an unrecognisable blob.
You can try to use SID (Super Image Denoiser) with quality “SUPER” and enabling volumetric. It give better result than other denoiser for pictures with important volumetric.