This last weekend I had the opportunity to photograph the "Moonbow" at Cumberland Falls State Park in southern Kentucky. This is one of the few places in the world where you can witness a rainbow at night and the only place in the Western Hemisphere.
Each month there is a 4-5 day window around the full moon calendar where if the sky is clear, a rainbow will appear as the moonlight shines through the mist. When you witness it in person, you can only see a white arc but your camera will capture the color which can be viewed on your LCD screen. For each cycle of the full moon, the window of time will vary from late PM to the early AM hours. On this particular night I was there until 2:00 am and the moon bow was still going strong when I left.
If you plan to photograph this rare feature, be prepared to arrive early or several hours before it gets dark. There are 3-4 different locations along the falls where you can set up to capture the moonbow. I recommend bringing the widest lens you have if you want to capture the moonbow and the waterfall in the same image. I was using a 16-35mm lens on a crop sensor camera and could not find a location where I could fit it all in.
A wide open aperture is almost necessary to capture enough light on the image. 30 second exposures still came out on the dark side and required large adjustments in post-processing. I recommend setting the focus while there is still enough light using auto-focus and then switching it to manual focus. Once it gets dark, it will be difficult or impossible to use the auto-focus and figuring out of your picture is sharp is a guessing game.
I recommend this exciting photo opportunity to anyone who is in the area of Cumberland Falls, it is truly a unique experience.