Hasta La Motion!

I’m a littlebit lost. I tried to "google the question" but no luck.

XSI rendered a decent depth map. I’d like to apply a DOF effect by zblur. BUT I need the opposite of the behaviour of zblur. So I need an inverted alpha channel. I tried the "-depth" and then all of the math. No luck.
Then I tried to invert the depth map, but as the black turns to right I had to add a mask to it to exclude the unnecessary area. Little Grade then, and the depth map looked all right, but nothing changed.

What am I doing wrong?
Or is there any alternative way? :radar:

Thanks in advance!

rich