Ok maybe I didn't explain that as good as I thought I did, so here's another little explanation on that subject.

This a very very simple scene 3 objects we want the middle box be the focus object in there ok.

Let's do everything I told you with the camera in this particular scene the first object is 142 units away from the camera, the middle one at 231 and the last one 350 but the wall is at 384 units so we mesure everithing up. Good lets render and use this values in the Z-depth dialog. and render.

Cool now we got the 2 passes the difuse and the z-depth, now use the Z-Depth as a chanel for the difuse render and aply the lens blur, but before that select the Eyedropper tool in photoshop and see what color your focused object is, mine is 168 so I use that for the lens filter.

That's all. play a little bit in there with the params until you are happy and I got this.

Finally some test with diferent focus color

Back object fucosed ...............................................Front object Focused