ART IN THEORY- Shadows in Paint

The artist must treat shadows differently than the lights. The greatest degree of value changes in nearly any environment will occur in the lights -- the shadows will be compressed into a much smaller range of the value scale you've established in your painting. Jacob Collins' tells his students, while painting, to keep in mind the phrase "Active lights, passive shadows," and to dwell on it almost like a mantra.

Rembrant - Man in Oriental Costume.

Rembrandt displays in his paintings masterful execution of active lights and passive shadows. Notice how when you moderately squint your eyes the shadowed area of the painting, including the man's hand, optically group into a massive single value -- this is because Rembrandt was acutely aware of how to compress shadows into a small value range in order enhance the illusion of light and form. The viewer would need to squint their eyes more extremely in attempt to view all of the lights as a single massive value -- this is because Rembrandt used a much larger range of values in the light areas.