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Exposures for Moonlight Photography

Overview

Taking a photo of a scene that is only illuminated by moonlight requires a long exposure and best done around a full moon. There is a fairly significant drop off as the days go away from a full moon. I’ve compiled information to give you a starting point for an exposure time.

See the interactive mobile calculator at: mkaz.com/light

Below is a rough chart based off the same data, should give a starting point for estimating moonlight photography exposures. Note, this is taking a photo of an subject illuminated by moonlight and not a photo of the moon itself.

If you wish to photograph the moon itself, the exposure is much different. The moon is an object reflected in sunlight, so the rule-of-thumb is to use the “Sunny-16 Rule”. This says, for an object in direct sunlight expose at f/16 and time is 1/ISO. So for 100 speed film, your exposure would be 1/125.

Remember: experience, practice, patience and bracketing will help all shots. Take good notes. I welcome your comments, experience or tips on improving this information.

Subject in Moonlight – Static Chart

The following exposures are for f/8 using ISO-100 slide film. See the dynamic form below for different aperture and film settings. The exposure times are rough starting points, the subject surface (reflection), ambient light and other factors will effect your exposure. When in doubt, bracket.

Moon Phase Days after Full Adj. Stops Exposure Time
Full Moon – – 8 mins
Gibbous Moon 3-4 days after 1.5 stops 22 mins
Quarter Moon 7 days after 3.5 stops 1 hr 30 mins
Crescent Moon 10-11 days after 6.5 stops 10+ hours
New Moon 14 days after 13.5 stops Not Practical

Note: The moon phases are reflexive, so 3 days after a full moon is the same as 3 days before a full moon.

Sources for Full Moon exposure:

Here is the Kodak Recommendation from Trial Exposures Under Difficult Lighting Conditions

Moonlit landscapes: 30 seconds at f/2 at 100

For scenes lit by the unobscured full moon to show full detail in the surroundings, not including the moon in the picture.

Adjusted: ISO 100 film at f/8 is 8 minutes

Full Moon. f/8, 4 mins, Fuji 64tAnother rule of thumb is the Looney 16 Rule, which says the full moon is roughly 250,000 times dimmer than the sun, which works out to 18 stops less light than full sunlight. (2^18 = 262,144) The sunny-16 rule for full sunlight says for ISO 100 film, expose f/16 at 1/125, so adjust this 18-stops.

Adjusted: ISO 100 film at f/8 is 8 min 30 seconds

So I used the base exposure for ISO 100 film at f/8 as 8 minutes, since 30 seconds of 8 mins is only a difference of 6% which is a small difference overall.

Calculating Luminance for non-Full Moon

Keep in mind that the days right around the full moon is brightest and then it falls off very quickly. [See the Chart: Earth Surface Lunar Illumination] Also published on the LunarLight site, is the following table which shows the calculated LV values for the above stated moon phases.

Adj Stops LV Situation
13.5 -17.00 Dim Starlight
6.5 -10.00 Crescent Moon: a=135, k=.2, z=60, d=nominal, LV=-9.51
3.5 -7.00 Quarter Moon: a=90, k=.2, z=45, d=nominal, LV=-6.38
1.5 -5.00 Gibbous Moon: a=45, k=.2, z=30, d=nominal, LV=-4.43
0.0 -3.50 Average Full Moon: a=4, k=.2, z=15, d=nominal, LV=-2.76

Reciprocity Failure

Reciprocity Failure is a problem that occurs with film’s ability to evenly measure light during long exposures. The characteristics of film is that during an exposure it is initially very sensitive to light but as exposure time increases the film’s ability to record light is diminished. So what a light meter may tell you should be a 1-minute exposure, for a particular film that exposure may actually need to be 8-minutes.

Black and White films are effective more by the reciprocity failure than color or slide film. Check your film data sheet for it’s characteristics and tables. Data sheets uses were collected from numerous sources (see below).

Unfortunately not all sources publish the same data, most sources have incomplete data for films and even manufacturer data seems suspect especially for very long exposures. All the best efforts were put in to give reasonable values.

Here are the Reciprocity Tables used for the calculations. Linear interpolation used for in between values. Please let me know if you have corrections, suggestions or more data available.

Addendum: Stop Calculations Explained

A stop is a doubling of light. So opening an aperture up by making it wider by one stop will half the amount of exposure necessary. So for example if you needed a 4-minute exposure at f/8, you would only need a 2-minute exposure at f/5.6. Here is a sample table of different exposure and aperture times, all of these have an equivalent luminance. Starting with a exposure f/8 at 1-minute, look at the range of exposure times based on changing the aperture value.

Aperture Exposure
1.4 2 sec
2.0 4 sec
2.8 8 sec
4.0 15 sec
5.6 30 sec
8.0 1 min
11.0 2 min
16.0 4 min

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