Depth of Field sets out to simulate camera focus effects. For instance in macro photography u'll see the photographer has its main subject (for instance an insect) quite sharp in view, yet the background has all blurred because its not in focus.

I think focal length is the distance from the camera to the point where the focus is perfectly sharp.

Lens radius would probably control how fast objects get blurry when they are further away from the point where objects are in perfect focus.

Set to current selection adapts the DOF settings to such extend that the currently selected objects in your bryce scene are in that perfect focus area.

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