Forgot to say to ignore the sky and the ground.
After a few of Blender Unit, all starts blurring. I checked the focus out of the camera and other settings that would have blurred the scene along the distance, but there is nothing turned on. I think that there are too many beams of leaves and too few pixels. How can I improve my scene? It’s strange, isn’t it?
Don’t look the ground and the sky. I put the cube far away from the camera to test if any “focus” option was turned on.