Tuesday, June 21, 2011
The Rarest Light
Look for the gap and the glow, and you’ll find light that transforms a scene into something extraordinary
For 18 years now, I've been specializing in wilderness landscape photography. During that time, I've been privileged to photograph many beautiful scenes, but on only a dozen occasions have I witnessed the rarest light of all. In my experience, truly extraordinary light occurs in two ways. In the first, the sun finds a narrow gap between dense clouds and the horizon precisely at the moment of sunrise or sunset. For just a minute, maybe only seconds, a beam of saturated light blasts through the gap and ignites the subject with the most colorful natural light you'll ever see. In the second situation, the sun, while still below the horizon, paints a broad bank of clouds with richly colored light. These clouds, which can be inside the frame or out, now become the dominant light source. While glowing clouds are beautiful, of course, it's the light those clouds bounce onto the landscape below that's truly remarkable. Like a giant softbox in the sky, the colorful clouds illuminate the landscape with a warm, soft, yet still directional light. The photograph seems to be suffused with an ethereal glow.
To understand how these two situations produce such unusual light, it helps to know a little bit about atmospheric optics, the science of how sunlight interacts with our atmosphere.
The sun emits light at all wavelengths, which means all colors. When sunlight strikes our atmosphere, however, it begins to interact with air molecules in a process called Rayleigh scattering. Blue light scatters much more strongly than red light, which is why the sky is blue on a clear day.
The amount of scattering is dependent on the distance the light travels through the atmosphere. At noon, the path length through the troposphere (the dense layer next to the Earth) is relatively short, roughly 11 miles or so. Enough blue light scatters out of the beam to make the sky blue, but the remaining light is still a mixture of wavelengths that we perceive as white. At sunrise and sunset, however, sunlight takes a much longer path through the troposphere because the light is traveling obliquely through the atmosphere, along a tangent to Earth's surface, rather than along a path perpendicular to Earth's surface. For example, if you're photographing an old barn in Kansas at sunrise, the path length through the troposphere is roughly 235 miles. At sunrise or sunset on a clear day, most of the blue light scatters out of the beam. The reddish light passes straight through, giving you warm light on your subject.
The exact numbers aren't important, but the principle is: Tall mountains that tower over nearby plains can get amazing light, particularly if the sun finds a narrow gap between dense clouds and the horizon.
The most vivid red light I've ever seen has occurred in exactly that situation, but "gap" photos aren't confined to the mountains. To understand gap photos, in general, let's first consider what happens during a cloudless sunrise. When the sky is clear, the colorful light coming directly from the rising sun is diluted by the bright, white light from the sky around the sun. It's like pouring white paint into red paint—the result is usually a pastel color that's pleasing, but not extraordinary. If the sun finds a gap between dense clouds and the horizon, however, the clouds block that bright, white light from the sky around the sun, and the result is some of the most spectacular light you'll ever see.
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