mental ray for Maya motion blur
In mental ray for Maya, you can choose between Linear and Exact Motion Blur. Motion Blur in mental ray for Maya blurs everything: shaders, textures, lights, shadows, reflections, refractions, and caustics.
The Linear mode results in instance motion; shape changes are not considered. The Exact Mode additionally exports motion vectors for every vertex of the moving object. Use it to motion blur objects with deforming shape. This mode requires more translation and render time.
The linear mode just exports instance motion performed on transform nodes, any shape changes (even linear movements) are not considered.