You can use this simple 3d Studio Max scene to practice your mental ray rendering with depth of field. There are numbers placed around the scene at escalating distances from the camera. There are numbers at 2, 4, 8, 16, 32, and 64 feet from the camera. Try modifying the camera’s f-stop value and target distance to see what kind of effect the blurring has on the different numbers. The checker pattern on the back wall is exactly 1-foot by 1-foot squares.
This scene was used in my mental ray depth of field quality tutorial.