Curiosity question about graining

reserv888

When I render (Iray) an image I ususally let it render until the graininess is gone.
Why is it always skin and hair where it disapperas last? 
Graininess on clothes and environments disappears long before the skin is grain free (especially the shadowy parts under the chin).

I understand that shadowy parts can take longer to render but why only the skin and hair and not the clothes?
 

Richard Haseltine

Because, as I understand it, those are the areas most likely to use things like SSS (subsurface scattering), where the light enters the material and is scattered in a complex way before emerging - that adds an extra level to the claculations compared to relatively predictable bounces from hard surfaces and so takes longer to converge.

reserv888

Thanks Richard!