Human skin has pores which are best represented by a bump map. Without the bump map the skin appears too shiny and smooth. Scattering will fill in the tiny crevices between bumps and make them appear less harsh. The effect is subtle, just like subsurface scattering itself, but important.
The bump map itself was created in Photoshop by filling a layer with 50% gray and then applying a Noise filter at an amount of 1%, Gaussian Distribution, and MonoChromatic. The result is a clean histogram with a bell shape fall-off to either side of middle gray.
Skin Specularity and Specular mapping
The SSS Fast Skin shader has two specular layers. The Primary Specular, used for broad and soft specular highlights and the Secondary specular which are much narrower and used for detailed sheen. Setting the Overall Specular weight to 1.0, to turn specularity back on and setting Specular Weight #2 also to 1.0 produces the following oily look.
Human skin however is not uniformly shiny. Where the skin is closer to the bone, it tends to shine more and also around moist areas, such as the lips. Specular maps are used to control skin sheen, with white turning shine on, and black turning shine off. A specular map for the Specular Weight #2 channel was made so that the lips look wet and the rest of the skin has only a slight natural oily shine.
In the Specular Weight #1 channel a noise map is then used to soften and further control the overall specular effect.
The Epidermal scatter color channel is used for colour mapping and a grey scaled version of the colour map is placed in the Unscattered diffuse color channel to control the diffusion of tones. Some of the skin colour comes from the diffusion of light in the reddish Subdermal layer. Here lipstick has been applied to the mouth and a little pinkness is added to the overall skin colour.
Raytracing the SSS Fast Skin shader produces a lot of light rays and can be computationally expensive. Turning on Only Reflect Environment turns off raytracing but still enables environment reflection. Here Reflection Weight is set to 0.1 and Reflection Edge Weight set to 1.0.
Setting a high Reflection Edge Weight ensures that the skin behaves realistically in that it reflects more when viewed from the side. As a lot of reflection was occurring around the edges of the inner lips, so a map was placed in the Reflection Edge Weight channel to prevent too much lip reflection.
Final Gathering adds that extra touch of realism by adding a little extra light and softening of the skin. Here Samples are set to 100, Maximum radius to 10, and Minimum radius set to 5. Scatter indirect illumination, under the Advanced Options needs to be turn on, as it is turned off by default to save render time.
The final result is a subtle skin tone with all the complexities of nature.
Here are the maps used
Andersson, Z., Mental ray fast subsurface scattering tutorial, Berlin, Mental Images, 2004
Alton , J., Painting with Light, Berkeley, University of California Press, 1949