The Oil Painter's Bible - chapter 5
Of equal importance to the principles governing spatial recession is an understanding of the nature of
light and shadow. Three-dimensional form is indicated by the distribution of light. Shadow is, in theory,
the absence of light, but in reality there is light in shadow as well. Areas of light are generally illuminated
by light rays which have traveled in a straight line from the strongest source, whereas shadows receive
their light indirectly, as the rays ricochet off nearby surfaces and bounce in behind objects that block the
direct rays, or from secondary light sources weaker and/or farther away than the primary light source.
Without secondary light, shadows would read as black. The dark side of the moon is an example.
Secondary light is what enables us to perceive form within the shadows.
The shadow is darkest at the just beyond the planes illuminated by the primary (strongest) light
source. This area is called the shadow accent (sometimes referred to as the core shadow) of the body
shadow. The phenomenon of the shadow accent is best understood in scientific terms. Whenever enough
light is present to allow us to see at all, there are light rays coming from many directions, of varying
strengths, both reflected and direct, often from more than one source. The strongest light creates what we
consider light and middletone areas, whereas the weaker are only visible in shadow. Whatever is blocking
the strongest light also blocks a certain amount of secondary light, and the closer the shadow is to the
blocking obstacle, the more of the secondary light rays are blocked.
A good analogy is to suppose we are standing under an awning or umbrella in the rain. Our head stays
dry, being closer to the awning, umbrella or whatever is blocking the rain. Further down, some rain may
reach us, where there is greater distance between a given surface and the awning. If we substitute light
rays for rain, it becomes apparent why the shadow accent is darker than the rest of the shadow. Less light
means more dark. Beyond the shadow accent, more reflected or secondary light is allowed to enter the
shadow area, reflecting off nearby objects or planes, or from secondary (weaker) sources, as the distance
from the light-blocking obstacle increases. This light is commonly called reflected light in shadow,
although it often includes direct light from weaker or more distant sources as well. Perhaps a more
technically correct term would be secondary light. So the shadow accent, also known as the core shadow,
is most simply described as the zone of shadow between the primary light and the secondary light.
The shadow accent is a most useful device for describing interior planes, that is, planes within the
edges of the object being depicted, by its shape and by how sharply it makes the transition from
middletone to shadow. A sharp change in angle will have a sharp transition; a more rounded form would
have a softer transition from middletone to shadow. Once the artist understands this principle, he or she
will look for the shadow accent, and will use it to good advantage.
The cast shadow follows the same principle as the body shadow (the shadow on the unlit side of the
object in question), in that it is darkest at the edge nearest whatever is blocking the light. Its edge is also
sharpest at that point, and softens as it recedes from there, due to reflected light from surrounding planes
or from secondary sources. The middletone is the area on the lighted side which is far enough from the
angle of incidence of the light rays to our eyes that the body color of the object is least altered by the light.
Thus the color of a given surface is seen at its highest chroma, or intensity, somewhere in the middletone.
This is explained in greater detail in the chapter on color.
The highlight is the point at which the light from the primary light source bounces off the object and
to our eyes the most directly. It will contain more of the color of the light source than any other area in the
light. The highlight will describe the surface texture of the object being viewed, by the degree of
sharpness at its edge, and by the contrast between its value and the value of the middletone. There will be
a transition zone between highlight and middletone. The extent of this transition zone, again, depends on
the texture and shape of the surface, and on the intensity of the light from the source, striking at the
highlight. It is necessary at this point to address the way in which color is affected in the shadow areas.
Shadows are areas where the direct light rays from the strongest source cannot reach. If there were but
one source of light, and no surfaces nearby to reflect light back into those areas, the shadows would be
totally dark, and we could not read shapes within them. However, situations such as that are rare, except
in outer space. In reality, there is usually more than one light source, and/or nearby surfaces which reflect
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