I searched the Internet for hours without success. I am able to create an Animation and to add the depth of field to the rendered Scene.
To overlay it with a real Video I created a plane which texture is the Video. When I blur now by using the depth of field effect, the edges of the 3d object are not blurry beyond the edges and do not appear to be mixed with the Image in the Background. The sharp Image of the Video is displayed till the edges of the object before the dof effect. The plane is touching the TM letters and the Focus is put on the plane.
Somewhere it was written that blurring of textures does only work with UV mapped Images but UV mapping the Video on a plane does not Display the Video in once but subdivided into the 2 triangles which are forming the plane.
Any expert here? ;)