Light is that energy that shapes the forms and its absence creates the shadows.   It is what draws us into a painting.   The contrast of the adjacent darkest and lightest values, known as chiaroscuro, is easy to paint for most artists.  More difficult is the middle range – those values that transition the light into the shadows crossing a wide range of cool and warm hues.  There appear to be infinite possibilities, yet as artists we choose to minimize the visual transition for the viewer.  Mastering the art of this transition may take a lifetime.  It is the most important aspect of painting.  It is the ‘holy grail’ of artistic endeavors.