Fill light

The same two equal incident strength sources placed on opposite side of a face will work to cancel each other out creating overall dimensionally flat appearance with dark unfilled voids in low areas neither light reach.

The Highlight:Shadow ratio convention long used in portraiture is based indeed on the reflected light the camera records when the sources overlap.

A systematic approach for visualizing the cause and effect is to start from a baseline set-up with the testing done outdoors at night or in a large darkened indoor space where there will be no reflected "spill fill".

Set a camera on a tripod at the aperture desired for depth-of-field and start with just the fill source located centered and chin level to a subject facing the wearing black and white textured clothing.

The nose on front of the face will be observed to be lighter than the ears due to the inverse-square fall off of the fill source and it will create a light-to-dark gradient.

With most recording media (B&W negative being the exception) the white clothing will not be correctly exposed when the fill light is adjusted based on shadow detail.

That problem is solved by adding the key light which for this exercise should be placed 45° above the eye line and 45° to the side of the nose of the subject facing squared to the camera.

The combination of overlapping key over centered fill works together to change scene contrast to exactly fit the dynamic range of the recording media.

Due to the way the photographic process is engineered the response between the black and white extremes in the scene range will be reproduced in a way similar to human perception and the image will look "seen by eye normal" in the flash lit foreground.

Repeating the same tests indoors where light bouncing around the room is a factor will make the photographer aware of how the shooting environment will change both the appearance and numerical ratio the same standard "baseline" set-up produced when there was no "spill fill".