How Do I get Depth of Field working together with 2D Toolkit sprites?

I have a scene containing sets of sprites made with 2D Toolkit. There's a foreground and a background, set far back on the Z axis. It is a perspective camera. All I want to do is keep the foreground sprites sharp while blurring out the background sprites.

However, whenever I try to use Depth of Field, there is no value that allows me to do that. Setting the target focus to a foreground sprite does nothing. If I try to adjust it using Visualize, the entire screen is always affected; as if it doesn't seem to register the differences in Zdepth in all of the game objects.

Does anybody have an idea of what might be going on?