How would I make a modified Depth Of Field Image effect like a ramp shader?

I need to make an image effect that makes things blacker as they are closer to the player and whiter as they are further away. Depth of field image effect is close, except instead of blurring and unblurring I need blackening and whitening. How do I go about this? I thought about modifying the depth of field script provided with unity but I can't make heads or tails of it.

Is there a tutorial out there for this sort of thing or a handy API?