The Matcap projection type will correctly apply Matcap textures onto surfaces. Connect a RGB Image node containing the matcap texture into the desired material channel, and wire the Matcap node into the projection type (figure 1).
Figure 1: A Matcap projection node used to correctly project a matcap texture connected to the DiffuseAmount of diffusion, or the reflection of light photons at different angles from an uneven or granular surface. Used for dull, non-reflecting materials or mesh emitters. pin on a Diffuse materialUsed for dull, non-reflecting materials or mesh emitters. node.
Figure 2: The node network associated with the results in figure 1