Good photographs are found at the crossroads of light and shadow. An unrelenting sea of light appears flat and lifeless. Ask any passionate photographer; the most beautiful times of day are dawn and dusk when shadows are long, offering contrast, refuge, shape and form. Shadows breathe life into the world and add a brilliant sparkle to luminance; light can't be fully appreciated without darkness. Renaissance painters knew this well and developed a technique called Chiaroscuro (Italian for "light-dark") to dramatically model three dimensional form with light. Ironically, many of the books that have aided my photography over the years have been treatises on painting. Despite the many differences between painting and photography, photographers can learn much from the master painters of the past.
Rembrandt van Rijn illuminated his subjects from above and to one side. This gives a very distinct look with the side of the sitter's face closest to the light source bathed in light and the opposite side largely in shadow. A triangular highlight below the eye on the dark side of the subject's face accentuates the contours of the cheek. Rembrandt kept his color palette limited, drawing further emphasis to the prominent contrast and nuanced lighting.
Though stylistically different from Rembrandt, Leonardo da Vinci's treatment of light is no less masterful. His most famous work, the Mona Lisa, is another ingenious example of the transformative power in light and shadow. Most of the painting's contrast is conveyed in the form of light, not color. It is interesting to note that the subject's hands are modeled in light with as much precision as the face; no detail is too small to be irrelevant.
Perhaps history's greatest lesson to the modern photographer is this: No exotic sensor design, blazingly fast autofocus system, extreme megapixel count, dynamic range capability or impressively large f-stop can overshadow the photographer's skill in searching for light or creating interesting light. We live in a time when we have everything, yet we will never be able to transcend the same fundamentals of art mastered by those who came long before us.