During a full moon with clear skies, there is approximately 5.5% as much light as a sunny day. Using the Sunny 16 rule, we can calcuate a basic full moon exposure as follows:
- 125 ISO at f/16 on a sunny day, use a shutter speed of 1/125th
- Opening the aperture 2 stops to/8, the shutter speed is 1/500th
- There’s 5.5% as much light as daytime under the full moon, open up 18 stops for an exposure that is primarily lit by moonlight (1/250th is 1 stop, 1/125th is 2 stops, 1/60th is 3 stops, etc.) – the baseline full moon exposure is 8 minutes at f/8, ISO 125
- Adjust your exposure as necessary based on the desired aperture, exposure time, ISO settings, and reciprocity failure. You can read about digital exposure testing in this previous article.
There are a lot of factors that contribute to good exposures by moonlight. One of the simplest data points is how full is the moon? As long as you’re within a few days on either side of the full moon, there is enough light for reasonable exposure times. If the moon is 90% full the night you’re shooting, you’ll need more exposure than the night it’s full.
The next full moon is on Wednesday, February 20th, 2008. Let’s take a look at the amount of moonlight, and sunset and moonrise times in California this week (courtesy of the U.S. Naval Observatory):
- Sun Feb 17th – 87% moon – Sunset 5:32pm, Moonrise 2:01pm
- Mon Feb 18th – 94% moon – Sunset 5:33pm, Moonrise 3:12pm
- Tue Feb 19th – 98% moon – Sunset 5:34pm, Moonrise 4:21pm
- Wed Feb 20th – Full Moon- Sunset 5:35pm, Moonrise 5:27pm
- Thu Feb 21st – 99% moon – Sunset 5:36pm, Moonrise 6:30pm
- Fri Feb 22nd – 97% moon – Sunset 5:37pm, Moonrise 7:30pm
- Sat Feb 23rd – 92% moon- Sunset 5:38pm, Moonrise 8:30pm
Notice the sunset and moonrise times are very close on the night of the full moon. The moon rises before dark on the days before the moon is full. On these earlier moonrise nights you can usually start shooting by moonlight as soon as it’s dark. On the nights after the full moon, moonlight shooting doesn’t start until later – typically an hour after the moonrise time (depending on the terrain and cloud cover).
Watching the moonrise schedule becomes particularly important during the Summer months when the daylight hours are longer. During the next few days you can begin shooting by moonlight a little bit after 6pm. In July it’ll be 9pm. Two days after the July full moon you wouldn’t have enough moonlight until 10:30pm (about an hour after the moonrise).
Wherever you are over this holiday weekend, I hope you’ve got a camera with you. Look for the moon, give your photos a little bit more exposure on Sunday and Monday night, and stay warm out there!