Here's a quick explanation of how it works. The box labeled (Lens Radius) is the "extent" or radius of your focus. A value of 0.00 being "Deep Focus", ie: everything is sharp, just like normal Bryce render. The higher you set this number, the less focus radius you will have. I find that a value of 0.01 to 0.04 gives the best effect, a value near or over 1.00 will give the most dramatic or "shallow focus". The box labled (Focal Length) is the distance from the lens plane to the epicenter of the effect. If you select the object (by clicking on it) that you want at the center of the focus, then click the button that is labeled (Set To Current Selection), the number in the Focal Length box will change to reflect the new distance. Note: You MUST check the dot labeled (Depth of Field) or there will be no effect. Also, the higher the "Rays per pixel" number, the better the effect will look. That's a quick explanation that should get you going. I plan of doing a tutorial on this effect soon. Years ago when I was an Assistant Cameraman, part of my job was calculating Depth of Field for different aperature and distance settings so I could focus the camera for the operator during a shot.