© 2000 Jeff Conrad
The moon often holds a fascination for landscape photographers. A rising or setting moon can add the final touch to an image, and in some cases can be the main attraction. Many photographers wonder about the correct exposure, forgetting that a landscape photograph that includes the moon is still a landscape photograph—unless the sky is very dark, determining exposure as if the moon were not present often works just fine. Several common myths make the process seem more complicated than it need be.
The moon is illuminated by the sun, so it’s sometimes claimed that a normal daylight exposure, in accordance with the “sunny 16” rule (f/16 at 1/ISO film speed), will be correct. Some photographers suggest slight variations, such as the “loony 11” rule (f/11 at 1/ISO). Unfortunately, things aren’t quite that simple.
The sun is indeed a constant light source, but the moon’s reflectance varies with its phase, and the amount of light absorbed by the earth’s atmosphere varies with the moon’s altitude above the horizon. Viewed from earth, the luminance of the moon is far from constant.
The reflectance of a full moon is approximately 15%, but is only about 8% one day before or after the full moon, and decreases to about 2% for a quarter moon.
Many published lunar exposure tables are for astrophotography, and using them for landscape photography usually doesn’t work. Astronomers like to photograph the moon when it’s high in the sky, but landscape photographers prefer to photograph it near the horizon. The closer the moon to the horizon, the greater the absorption of light by the earth’s atmosphere—just as with sunlight. This absorption of light, known as atmospheric extinction, has considerable effect—the difference between the moon’s luminance at an altitude of 40° and just above the horizon is nearly eight exposure steps at sea level. The absorption is less at higher elevations, and also varies with local atmospheric conditions, such as temperature and haze. The variation is greatest near the horizon.
The luminance of the moon is the sum of the light from the sky and the light from the moon, and the amount of light in the sky defines three different situations.
Between sunrise and sunset, exposure is easy—just expose for the landscape as if the moon were not there. The moon and the foreground usually will be rendered appropriately. The contribution of skylight to the total luminance often is much greater than that of the moon itself, especially when the moon is near the horizon, so that variations in the moon’s luminance have little effect. Because of this, “rules” that recommend a constant exposure for the moon may appear to work. They do so by chance, however; successful exposures usually happen to be the same as would be determined for normal landscapes using an exposure meter.
The foreground usually is rendered as a silhouette, the moon is the main determinant of exposure, and variations in its luminance have considerable effect. When the moon is near the horizon, the effect of atmospheric extinction is particularly apparent, and most popular ”rules” result in severe underexposure.
Determining exposure with a meter can be tricky; an ordinary broad-angle exposure meter will give excessive weight to the dark area, and indicate settings that result in severe overexposure. With a very long lens, a camera with built-in spot metering may be able to read the luminance of the moon by itself. Of course, with a dark sky and foreground, a tiny moon isn’t very interesting, so a long lens usually makes for a better image.
Measuring the luminance of the moon with a handheld meter is difficult; because the moon has an angular diameter of approximately 1/2°, even a 1° spot meter won’t suffice. Measurement is possible with specialized luminance photometers (such as the Minolta LS-110, which reads 1/3°), but such meters are very expensive, and usually indicate luminance rather than f-numbers and shutter speeds. For most photographers, luminance photometers aren’t practical.
When the moon’s luminance can’t be measured, it can often be estimated, and exposure determined accordingly. Table 1 gives the approximate luminance of the full moon at various altitudes above the horizon.
A twilight sky often makes the most interesting background; it’s dark enough to afford a sense of evening, while still providing some illumination on the foreground, and sky color can be spectacular. The best exposure usually is based on the landscape, but may require some adjustment from the meter indication. As the sky grows darker and the moon moves higher, the contrast between the moon and the foreground can pose an additional challenge.
Exposure meters render whatever they measure as a middle tone. This works well for daytime landscapes, but is usually less effective for evening scenes. Often, the most effective way to convey the feeling of twilight in a scene facing opposite the setting sun is to control the rendering of the sky. Doing so usually also results in a satisfactory rendering of the foreground. A center-weighted averaging meter can often be used, but a narrow-angle meter is much better—and a 1° spot meter is ideal.
During the first few minutes after sunset, basing exposure on the meter reading usually is a good choice. When the pink in the sky has given way to the blue of the earth’s shadow, the rendering can be slightly darker, perhaps one step less than the meter indication. In the terminology of Ansel Adams’s Zone System, the sky would be placed on Zone V just after sunset, with the placement gradually reduced to Zone IV. By approximately a half hour after sunset, the sky is quite dark, and attempts to render the sky as other than black often fail to convey the impression of late evening.
Regardless of the moon’s luminance, the moon is only one element of a typical landscape photograph—with a 100 mm lens on a 35 mm camera, a full moon is slightly less than 1 mm in diameter, occupying only 0.3% of the image area. Unless the sky is very dark, and the moon is the only item of interest, the exposure usually should favor the landscape.
When the contrast between the moon and the foreground is more than about three exposure steps (perhaps four with negative film), an exposure that retains detail in both the moon and the foreground becomes difficult. If the exposure is based on the foreground, the moon loses detail, becoming a blank white disc. A small blank moon may be acceptable, but a large blank moon often is simply a distracting highlight.
A similar problem exists with contrast between the moon and the sky. When the contrast exceeds about three exposure steps, it’s difficult to maintain tone in the sky and detail in the moon. If the sky is to be rendered as middle tone, the workable contrast is only about two steps.
Determining the contrast requires knowing the luminance of the moon, and involves the same considerations as does photographing the moon at night. Again, a table can often provide sufficiently accurate estimates.
Sometimes a split or graduated neutral-density filter can be used to reduce the moon-foreground contrast, but it doesn’t help much with the moon-sky contrast. Another approach would be to make two exposures—one for the foreground and sky, and another for the moon—and combine them digitally. But it’s usually easier to choose a time when the luminance of the moon and that of the foreground are reasonably balanced.
In most cases, a day that affords the best balance between the moon and the foreground is one on which the moon rises just a few minutes before or after sunset. This is often the day of the full moon or the day before, but in some months, no day is really suitable. A moonrise before sunset is often better for a natural landscape, with a pink and blue sky; a moonrise after sunset may be better for a cityscape, allowing time for the building lights to be turned on. The day after a full moon is often a good choice for rendering the foreground as silhouette.
Using similar logic, the best time to photograph a moonset usually is the day of the full moon or the day after.
For unusual situations, such as photographing the moon rising above a nearby tall building, or setting above a mountain, the best time may be several days before or after the full moon.
Because of the earth’s rotation, the moon appears to move approximately half its diameter in one minute, so long exposures are best avoided. The longest acceptable exposure time is somewhat arbitrary, and depends on the amount of enlargement, but most guidelines recommend an exposure no longer than
|focal length of normal lens|
|focal length of lens used|
The shortest exposure time obtained using this formula allows the moon to move 0.05% of the film diagonal.
For a 35 mm camera, a 50 mm lens is considered normal, so with a 100 mm lens, the longest acceptable exposure time would then be 2.5 to 5 seconds. With a 500 mm lens, the time would be 0.5 to 1 second.
Calculated values for luminance should always be used with caution, but they’re often sufficient for planning a photographic outing. Table 1 shows the approximate luminance of the full moon vs. altitude at sea level, with a clear sky.
ISO 100 film
Altitude in Table 1 is the angle of the center of the moon above the horizon. Values include an allowance for atmospheric refraction.
Luminance values in Table 1 include only the light from the moon. When photographing the moon before sunset or after sunrise, the total luminance usually will be considerably greater because of the contribution of skylight.
Exposure values shown correspond to the meter calibration used by Canon, Nikon, and Sekonic. Minolta or Pentax calibration would increase EVs by approximately 0.16.
As Ansel Adams suggested in Natural Light Photography, it’s possible to measure the luminance of the sky near the moon, add the measured value to the calculated luminance of the moon and then calculate the exposure, but doing so is tedious and usually unnecessary. When the sky or foreground luminance nearly equals or exceeds the moon’s luminance, exposure is governed by the sky or foreground, so the luminance of the moon doesn’t really matter. When the luminance of the sky is three steps less than that of the moon, the increase in moon luminance from the skylight is only 1/6 step, and can be ignored. At that point, however, the contrast usually approaches the maximum that the film can accommodate.
When the sky is dark, the moon’s total luminance may be reasonably close to the values in Table 1. Exposing according to those values will result in a middle-tone rendering, often appropriate when a deep pink or reddish-orange moon is just above the horizon. As the moon moves higher in the sky, a lighter rendering often better conveys the impression of the moon as a bright object. But it’s largely a matter of personal taste.
Moon luminance decreases rapidly as the moon moves away from direct opposition to the sun. One day before or after a full moon, the luminance decreases by nearly 1 EV. The values in Table 1 are for a moon that is approximately four hours less than full; when the moon is exactly full, the values are about 0.3 EV greater. The moon is unlikely to be exactly full at the time of rise or set; more commonly its luminance is slightly less than the values in Table 1.
Atmospheric absorption is less at higher elevations, and the moon’s luminance consequently greater. Local atmospheric conditions can result in additional variations of as much as two exposure steps, even with a relatively clear sky. It’s a good idea to start with the values in Table 1 and bracket several exposures on either side.
An EV, or Exposure Value, represents a set of equivalent f-number and shutter speed combinations: all combinations of f-number and shutter speed with the same EV will result in equivalent exposures.
EV 0 corresponds to settings of f/1.0 and 1 second, or any equivalent combinations. A greater EV indicates less exposure; each 1-EV increment is equivalent to one exposure step, so that EV 1 corresponds to f/1.4 and one second (or f/1.0 and 1/2 second), and so forth. A table of Exposure Values and equivalent camera settings is included in most camera manuals.
The EV that will produce the correct exposure depends on the film speed and the subject luminance, so an EV for a given ISO film speed can also be interpreted as a measure of subject luminance. ISO 100 is usually chosen as a reference speed; camera manufacturers commonly specify limiting light levels for metering and autofocus in EV for ISO 100 film. Many handheld meters also indicate luminance in EV for ISO 100 film; exposure settings are then determined from a dial on the meter. If the film speed is other than ISO 100, the EV must be adjusted accordingly. On a meter that indicates EV, the film speed is usually set on the exposure calculator dial.
Because an EV corresponds to a set of f-number and shutter speed settings, it’s easier to work with luminance in EV than in candelas per square meter (cd/m2), as should be apparent from Table 1. Knowing the luminance in EV of two different scene elements also makes it easy to determine the contrast.
Meter calibrations vary slightly among manufacturers, but the differences are on the order of 1/6 exposure step—insignificant in the context of estimating the moon’s luminance.
At an altitude of 5°, the luminance of the moon is 561 cd/m2, equivalent to EV 12.13 for ISO 100 film. Possible camera settings could be read from a meter’s exposure calculator dial; one combination of settings would be 1/60 second and f/8. As with a meter reading, this would produce a middle-tone exposure; a more appropriate exposure could be achieved with settings of 1/60 second and f/5.6
If measurement of the sky indicated an EV of 9, the luminance difference between the moon and the sky would be slightly greater than three exposure steps, and the range would approach the maximum that color reversal film could accommodate. An appropriate exposure could be achieved with settings of 1/30 second and f/5.6.