Lunar Rainbows - When to View and How to Photograph a "Moonbow "

On a brilliant moonlit night in spring, a trip to the bridge at Lower Yosemite Fall offers an opportunity to witness one of the rarest and most special Yosemite wonders--a lunar rainbow, or “moonbow.” John Muir described the experience in his book The Yosemite: “This grand arc of color, glowing in mild, shapely beauty in so weird and huge a chamber of night shadows, and amid the rush and roar and tumultuous dashing of this thunder-voiced fall, is one of the most impressive and most cheering of all the blessed mountain evangels.”

I have a special appreciation for this awe-inspiring spectacle. It was the backdrop when my wife and I exchanged our wedding vows, and it has been the subject of my photographic efforts for over twenty years. A lot of water has literally passed under the bridge since my first trial and error attempts to capture its essence. Science and technology have caught up with me. Websites post the optimum viewing times and modern cameras make the task of capturing it much easier. Still, here are some tricks to getting optimum results.

Ten Tips for Photographing Lunar Rainbows

1. Plan ahead! Check the Texas State University website for timing. Their predictions are based on viewing the moonbow from the terrace on the east side of the bridge at Lower Yosemite Falls. Ideally pick a cloudless night, as haze and clouds greatly reduce the intensity of light, and the resulting moonbow. Keep in mind, the intensity of the light drops also off considerably the days prior to and following the full moon, resulting in the need for longer exposures.
2. Start fresh! If you are using a film camera, start with a fresh roll of film and batteries. You don't want to be changing film in the dark and damp. For my film cameras, I favor using Provia 100 or 400 ISO transparency film, which have fairly accurate color balance and far less reciprocity issues (color shifts and a fall-off in sensitivity during long exposures) than most other films I've tried. For digital cameras, make sure your batteries are fresh and set the ISO to a low noise setting, such as 100 or 200.
3. Look for your shadow! The moonbow will form an arc of 42 degrees around the anti-lunar point, which happens to be the shadow of your head.
4. Use a tripod! To ensure you get a sharp photograph of what can be a very fuzzy subject set your camera on a tripod. Though the sight of a moonbow has often rendered people motionless and speechless, images made with low ISO film and digital camera settings easily exceed one's ability to hold still for the duration of an exposure. ;-)
5. Choose your spot! You want to be out of the direct spray, but where the mist (which is where the moonbow is most visible) is in view. This helps in three ways. This position keeps you dry, keeps your lens relatively clear of mist and keeps your tripod steady during the long exposure.
6. Bracket your exposure! The basic exposure for a moonbow is the same as any moonlit landscape. The trick here is not to make the exposure too long because the moon is moving and the moonbow moves with it. If you remember Isaac Newton's experiments with light and a prism, he was able to split white light into the visible spectrum or red, orange, yellow, green, blue, indigo and violet and then recombine the colors to once again form white light. Well the same is true for the moonbow. If the exposure is too long, the colors recombine into white light as the moonbow moves. Bracket your exposure in 1/2 stop increments on either side of the basic starting exposure (1 minute at f-4 with 400 ISO). So with 400 ISO try 30 seconds, 45 seconds, 1 minute, 1-1/2 minutes, 2 minutes. If you are working in automatic mode, use your exposure compensation dial to lengthen or shorten the exposure. If your camera doesn't permit automatic exposures beyond 30 seconds, consider switching to manual mode and using a cable release on the “B” or “Bulb” setting to keep the shutter open. This has the added advantage of permitting you to lengthen the exposure in the event that mist suddenly changes direction and blows toward you. If it does, simply shield the lens with your hand (don't touch it) and keep track of the additional time you need to add to the exposure to compensate for the time your hand covered the lens.
7. Protect your camera gear from moisture! The easiest way is to use a small (13 gallon) trash-can liner. Poke a small hole in the bottom of the bag and push the lens through. The opening should be smaller than the lens diameter, so that the plastic has to stretch to get around the lens, hence forming a nice tight seal. Then pull the bag over your camera.
8. Bring a small flashlight and a small towel! Moonlit nights in Yosemite can be as bright as daylight, but a flashlight can help you see in the shadows on the hike to the falls, adjust the settings on older cameras, and help you inspect the front of the lens. The towel or lint-free lens cloth is used to dry the front surface of your lens between exposures. Excessive mist on the lens will degrade the quality of your image. Take care not to shine the light into the front of other photographers' lenses while they are making exposures!
9. Use manual focus! The auto-focus features on many cameras have difficulty focusing in the misty, low light conditions. If yours is balking, switch to manual focus. If you are having trouble focusing manually, spin around toward the moon and focus on it.
10. Dress warm! It is windy out there, and wet. It is hard to concentrate on photography if you are uncomfortable. To ensure your visit is enjoyable and productive, dress as if you were going on a long winter walk in the rain. Even after you have finished your photographic efforts, you are likely to want to linger and savor this truly special phenomena.

By the way, don't fret if you can't see the colors of the moonbow when you are standing in the wind-whipped spray of the falls. Many people can't. It depends on the number and responsiveness of color sensitive cones you have in your eyes. I happen to have very good night vision, and apparently a disproportionate number of color sensitive cones in my eyes, so the full spectrum is visible to me, but, I remember photographing it with a friend who couldn't make out anything but a white blur while I saw a rainbow as clear as it would be in the daytime. If you do everything right, your camera will record the full spectrum.

© 2006 Keith S. Walklet, QuietWorks Photography