Problem with defocus node

I have a scene with a large plane that should be ground looking like sand. What I want is to blur the back to give it depth of field. I turned on limits in the camera options and found from the start of the plane where the camera is to the end is 8.50 using the Dof Dist button. I set the Dof Dist to 4.25. What happens is I get a clear band at about the center, the plane and any objects before the center or after are blurred. What I really want is all object to be in focus up to 4.25 then anything after that blurred. I have tried using no zbuffer in the defocus node with various z values, and using the zbuffer but have not been able to get the results I was looking for. Am I using the defocus node wrong? Is there a better way to do this? Thanks.

I am not a blur wizard, but I think you can do it by using render layers, and putting everything that’s in the foreground on a blur free layer.

http://meanwhile.beds.ac.uk/wordpress/wp-content/uploads/2007/09/node.jpg

Everything on the Racoon layer stayed in focus everything on the MainItems had DOF applied.