The SSS and AO are fighting each other.
@spikeyxxx , maybe drawing an AO map could help. The AO map will tell Blender where AO should be applied and where AO should not. But I do not know how to make and use AO map, and never used it before.
But like you said, how does one know that…I will say that usually you want a hair emitter mesh unchecked at render time. That being enabled is probably causing the unwanted AO.
@theluthier , on my last post, I have stated two problems regarding the AO:
1) the AO can increase the intensity of the shadow, which under certain lighting condition is unwanted and ugly shadow.
2) the AO can generate a unwanted shade when a hair emitter mesh is used.
Hiding or unchecking the hair emitter mesh can only solve the 2nd problem. Even if no hair emitter mesh is used, the 1st problem remains unsolved.
It is difficult to find the true cause for the 1st problem, because the increased intensity of the shadow can also be caused by the increased depth of the mesh. Therefore, decreasing the depth (i.e. making the holes shallower and smaller, making the concave surface less concave, and making the convex surface less convex) can decrease the intensity of the shadow. Bad geometry can indeed cause unwanted shadow. This led me to believe that it was my poor sculpting skills to cause the unwanted and ugly shadow, rather than the AO.
To completely solve the 1st problem, in my opinion, the best solution would be “don’t use any AO”, or “only use AO after all sculpting process is finished and finalized”.
