Depth Buffer in Material Editor

Does anyone knows how to use depth buffer in material editor?
I want to use it to mask area that invisible to camera.

This node seems to calculate distance :

So I used pack/unpack nodes to render depth information into 32bit float. With another camera I calculate difference between Depth node and render target. But this idea seems to be silly.

In the render target settings there is depth buffer field, is it possible to use it?

Hope it makes sense.