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

I need to make an image effect that makes t$$anonymous$$ngs blacker as they are closer to the player and w$$anonymous$$ter as they are further away. Depth of field image effect is close, except instead of blurring and unblurring I need blackening and w$$anonymous$$tening. How do I go about t$$anonymous$$s? 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 t$$anonymous$$s sort of t$$anonymous$$ng or a handy API?