Enlarging the clear looking area in dof?

Despite my trying hard I still do not find a way of enlarging the area where it should look clear when using dof. The use of dof keeps clear (non blurred) only a very tiny area of the scene. How do we enlarge the area that we want to look clear?

To express it in another way: we do want to add dof in a scene. But we want the non blurred area distance be three meters instead of fifty centimetres… how do we achieve such a thing?