This depth of field option in Bryce is excellent, when I first tried it I was unhappy with the result because the whole render came out blurred. It wasn't till a few weeks later that I discovered from reading somewhere that the depth of field is determined by an object within the scene.
For those who are unaware..... select an object and click on the button 'Set to current position' in the render properties. Also, I have found that a lens radius of 0,05 is particularly good for this type of scene.... http://www.renderosity.com/mod/gallery/index.php?image_id=1573167 

When your scene has rendered, the object in question and all other objects in that plain will be in focus and all others will be blurred (dependant on distance or closeness of course)

It's fun finding new ways to add enhancements to your render.