More than meets the eye


When I took a quick look outside tonight thinking I was hearing rain (it was actually the bubbles in my Pepsi can, but that’s another post,) the moon was peeking through a thin layer of high cirrus clouds and producing an effect much like the above image. However, the shots I took just now don’t look like the image above, because that one was taken under rather specific conditions.

Before I get to that, let me point out that the nice, ominous stormy colors seen in that shot are actually a moon corona, seen incompletely. Ice crystals high in the atmosphere diffract moonlight in a manner similar to a rainbow in sunlight, but with some important differences. A rainbow only occurs directly opposite the sun (some say your shadow points to the rainbow,) because the sunlight passes into the raindrops, bounces off of the back, and reflects back to you, skewed by the angle it enters and exits the raindrop at, which is what breaks up the white light into its component colors. It is often said that the sun has to be lower than 42º in the sky to make a rainbow, but that’s not exactly true – it applies to standing on a planet that blocks visibility below 5-10º. You can, however, easily see a rainbow at noon while flying in an airplane if there are clouds beneath you, making a nice ring around the plane’s shadow, and some hikers and mountain climbers can see this when they’re just above the clouds. I once had the memorable experience, too brief for me to capture on film, of exiting a high cloudbank in a jetliner and seeing a sideways rainbow projecting from the cliff-face of clouds from which we’d just emerged.

Sundogs, moondogs, and moon coronas are a little different in that they’re deflecting off of the sides of the ice crystals as they pass through, rather than bouncing off of the back. So they surround the light source instead of being opposite it. You’ll notice that there’s an indistinct ring of the colors around the moon in the image; the orange is outside the blue, and both are muted by the low light levels. Because they’re interrupted by cloud densities the effect is much richer, appearing to be the colors of the clouds rather than a simple diffractive ring.

Now here’s why I didn’t capture the same effect tonight. The moon reflects a lot of light, more than most people think, and it overwhelms everything else in the sky for photography. Getting something surrounding the moon during a full nighttime shot is next to impossible – even something as seemingly bright as a moon corona. Film and digital simply don’t capture the range of light like our eyes do. So, you resort to tricks, like getting the shot before full darkness when the ambient afterglow light in the sky can illuminate the surroundings. Or in this case, by choosing the timing carefully to have the moon largely obscured by denser clouds. While it might appear that the moon is in a bare patch of sky, it is actually hiding behind a finger of cloud stretching diagonally from upper left to lower right – that’s what the dark patch is. The moon still shone through, but greatly reduced in luminance while the corona was less obscured. You’re actually seeing two layers of clouds here: the upper layer produces the corona, and the lower layer of fast-moving fluffier clouds are producing the shapes and misty appearance. Tonight, I didn’t have these thunderstorm remnants decorating the sky, so I had a choice: get a detailed moon at proper exposure with no corona visible, or get the corona but blow the moon way overexposed and without detail. Getting both was out of the question unless I wanted to composite two separate images, and I consider that cheating and rarely engage in such stuff.

So, if you’re wondering why your shots don’t always look like the ones you see in magazines, it might be because there are more factors at work than you realize. Balancing light levels within photographs can be very tricky, and from having experimented with some studio and “product” photography, I can tell you it takes no small amount of work to achieve the nice subtle effects – even the photo of the softbox rig at the bottom of this post took two lights to get the detail I wanted, and it’s not ideal – there’s only so much effort I’ll go into just to illustrate a blog post ;-)

At right, another image showing the moon corona more distinctly, but losing all detail from the moon – you’ll notice the streaks of the clouds as they blew across the sky during the long exposure. Both of these images, by the way, were taken the same night as the one from this post on composition. I hadn’t planned on doing night photography that evening, and in fact had a movie lined up to watch, but you learn very quickly when you’re pursuing nature photography that when the conditions are right, take advantage of them, because they may not return again soon. That impulsive change of plans when I saw how the sky was behaving added a few dozen varied images to my stock, ones I may not have had the chance at again for, perhaps, years. Don’t procrastinate!

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