Ray Tracing - Thin-Lens Camera (Depth of Field)

I'm hoping to come up with some better scenes to trace in the future, these
scenes are pretty lame!

Image 1: Lens radius .3, Focal Plane Distance 6
Focus is on the purple sphere close to the camera.

Image 2: Lens radius .3, Focal Plane Distance 11
Focus is on the green sphere in the middle of the scene.

Image 3: Lens radius .3, Focal Plane Distance 20
Focus is on the far sphere.