When using Frischluft Lenscare (FL Depth of Field) or similar plugins in Compositing Programs such as After Effects, to create Depth of Field on your renderings, lots of times you get bad looking edges in areas where the depth-delta is quite large.
Especially when you have a rendering with no cg-background, that will be composited onto a sky or similar backdrop. This can be fixed.
What you do to decrease these depth-deltas, is, you add a rectangular surface or any kind of object right behind your objects that are getting bad edges and set them to be 99.9999% transparent.
This way the render-engine adds them to the depth-buffer, but does not (or almost doesn’t 0,0001%) render them into the beauty channel.