Blur Distance with FOV?

I have a 3D forest environment with terrain and trees and I'm trying to blur out the distant background of my scene using the FOV settings in the Post-Processing Stack. No matter how much I try, I'm unable to get a soled blur effect at that distance without blurring much of the mid-ground as well. Basically, I would like the player to focus on the mid-ground and draw attention away from super distant map features. A similar effect is created in Dark Souls in some areas; very near to the camera is out of focus, the play area is clear, and things very far away are blurred out. Is there some way to get this to work? Thanks!