Navigate to Window > Rendering Editors > mental ray > Approximation Editor.
The Approximation Editor lets you create approximation nodes and assign them to surfaces. Controls are provided for creating, assigning, and editing Surface Approximations, Trim Curve Approximations, Displacement Approximations, and Subdivision Approximations.
The default setting for mental ray for Maya approximation (see also Derive from Maya (default approximation) settings). This setting uses the Fine displacement approximation setting. This advanced technique produces memory-efficient tessellations that capture extremely fine details in displacement maps. By default, the algorithm is set to use a view-dependent technique that tessellates details as small as 1 pixel across.
For each approximation node, there are various settings, including tesselation method and style, that determine how the renderer tessellates objects at render time. For information on how to customize these settings, see mental ray Approximation Editor nodes.