|Image: Crowded Comics single panel image by Kevin|
2. Focal shift
|Image: layouts for "A Magic Life" from Fables #113, written by Bill Willingham.|
In a camera, when you are zoomed in, your focal depth is very shallow; only your subject is in sharp focus, while the background is fuzzy. This is also true of the naked eye, but the instant that you move your eye off of one thing and onto another, you automatically focus on the new object, so you don't notice. The white halo in this situation replicates the focal shift as you move your eye from a foreground object to a background one.
|Image: pencils from upcoming chapter of The Replacement God.|
In dark environments, the equivalent rule works, in which you can drop out details in the darkness as they come close to a foreground character or object, so that there is a halo of black around the character.
Speaking personally, I felt that coming to realize this was extremely liberating from a layout perspective. Everything I'd learned in the last 20 years was still intact, but there was now one more thing that would allow me to make my panel layouts just that much more effective.