When using a coarse approximation of an object and imitating a smooth surface (by bending normals away from the geometry normal) you may find shadow artifacts.

To correct this try the following:

• Use fine approximation to fit the surface more precisely (see Approximation styles)
• Disable Casts Shadows for the object to avoid self-shadowing (see Render Stats in the Rendering guide)
• Use a separate shadow casting object to cast shadows onto other objects
• Adjust the Ray Offset attribute on a per-object basis (see mental ray Ray Offset)
• Use shadow maps with a Bias instead of raytraced shadows, if possible (see Shadow Map Attributes)