The Multiplane Downshooter - Frame-by-Frame Stop Motion: The Guide to Non-Traditional Animation Techniques - page 95

Fig 7.4 the range of the key light on an object on the top shooting plane that does not cause
shadows on the lower shooting plane.
Key lights can create deep shadows, so it is often smart to give some
definition to the shadow areas on three-dimensional objects with fill light.
This goes back to the three-point lighting topic covered earlier. The range
of the fill light is similar to the key light for the same reason: shadows. The
fill usually is placed roughly on the opposite side of the key light, so it can
complement the stronger key light. This fill light should be diffused and not
wash out the shadows the key light creates. Both lights can utilize colored
Fig 7.5 the range and placement of the fill light on a downshooter with dimensional objects.