dof blender 2.5 problem

hi! I hope I have posted in the right section …
I come from Sardinia, I use blender for a year and a half
here’s my problem: When I try to use the blender node with 2.5 (after setting the dof in the camera) a black circle appears in the render that change shape if forced to a triangular, it becomes a triangle.
How can I fix it?
excuse my English