The Moon and planets

Learning goal

You will be able to explain why the moon comes out in the night sky as a snow-white, detail-less disc, through its being a surface with no light of its own, lit by the sun. You will expose the moon not like the stars but as if photographing a landscape on a sunny day, and reach the right exposure quickly with a starting rule. You will calculate in which phase of the moon the craters and mountains look most distinct, how small a space it takes up in the sky and how long a lens this calls for. You will choose the time to balance the moon in the same frame with a landscape on the horizon, and if you combine separately shot frames, you will say so. You will know how planets look in a long lens, and why they stay small and shimmering. You will plan when you can show, together with the moon, the face and skin of a person standing in front of the full moon.

The question

After the Milky Way frame shot with the tracker, your friend waits on the same plateau for the full moon to rise. When the moon rises over the hills, they fit the long lens and shoot a few frames. Opening the frames, they see on the screen a hollow ring glowing in the middle of a dark background. Yet looking with the eye, the dark patches on the moon's face, even some craters, can be picked out. They put the camera in manual mode and don't know what to do. Why does the moon always come out as a white blob?

Context

In the landscape track's planning lesson you saw the phases of the moon, that its rising and setting times shift every day, and that the full moon rises as the sun sets (Lesson M.2). In the flash sync lesson you met the starting rule that, on a sunny day, sets the aperture to f/16 and makes the time equal to the inverse of the ISO value (Lesson 19.5). In the perspective and compression lesson you also learned that when shot from afar with a long lens, the background comes closer to the subject in front and grows (Lesson 12.4).

In the night sky everything is faint, except the moon. The moon is the brightest object in the sky and has no light of its own: it reflects the sun's light. So the moon's surface is actually a rocky landscape standing under the sun in daylight. An exposure set for the night sky burns this daylight landscape snow white. This lesson takes up photographing the moon and the planets according to their own brightness.

Simple explanation

How is the moon exposed?

The camera's metering looks at the black sky covering most of the frame, tries to raise the average and burns the small, bright moon. That is why, for the moon, you need to switch metering to spot metering or set the exposure by hand. The Looney 11 rule is a simple rule that gives a starting exposure for the full moon: the aperture is the number in its name, and the time equals the inverse of the ISO value; for example, at ISO 100, 1/100 second. This is very like the starting rule of a sunny day, because the moon too is a surface lit by the sun.

As the moon's phase shrinks, the surface's light decreases and the exposure must be raised by a few stops; a thin crescent asks for clearly more light than the full moon. After the first frame shot with the rule, looking at the histogram and the magnified screen shows that the brightest regions haven't burned out. When photographing the moon the stars and the sky stay jet black; this is natural, because the moon is thousands of times brighter than they are.

Where is the detail, and how large?

The terminator is the line separating the moon's lit and dark halves. Near this line the sunlight falls on the moon's surface very slantwise; the shadows of craters and mountains lengthen and the surface looks embossed. At full moon the sun lights the moon straight from in front, the shadows vanish and the surface looks flat. That is why craters are best picked out in the crescent, first quarter and last quarter phases, along the terminator.

Angular diameter is the angle a celestial body takes up in the sky; the moon appears at an angle of about half a degree. That is why the size of the moon's image on the sensor is roughly a hundred-and-tenth of the focal length: around two millimeters with a two-hundred-millimeter lens, around five and a half millimeters at six hundred millimeters. On a thirty-six-millimeter full-frame sensor this is still small; filling the frame with the moon calls for a very long lens or a telescope. A moon near the horizon looks more shimmering and soft because it is behind a thick part of the atmosphere; as it climbs in the sky it becomes sharper.

Landscape, planets and people?

The best time to show the moon together with a landscape is twilight, when the moon has just risen from the horizon or is about to set and there is still some light in the sky; at that moment the brightness of the landscape and the moon come close to each other. Shooting from afar with a long lens shows the moon large behind a hill or a building. In the dark, on the other hand, the difference between the moon and the landscape is very great; combining separate frames is possible, but it is a combination and is told.

Planets are very small bright points even in a long lens. Around Jupiter its four brightest moons can be picked out as small points in a line; seeing Saturn's ring often calls for a telescope. A person standing far away in front of the full moon turns into a jet-black silhouette when the exposure is made for the moon; this is often a strong frame. If you also want to show the face and skin, you need to shoot at twilight, when the moon is near the horizon: the light still in the sky lights the face, and making the metering for the face, so that light skin doesn't look pale and dark skin doesn't look gray in this light, keeps the person in their own tone even if it brightens the moon a little.

Visual evidence

On the next full moon evening your friend waits on the plateau, facing the hills where the moon will rise, with a six-hundred-millimeter lens. As the sun sets the moon rises over the hills; the sky is still light blue. They place Selin, standing far away on the ridge of the hill, in front of the moon.

The first frame is at twilight: the moon orange and large, Selin's face readable in the sky's last light; they make the metering for her face, her dark brown skin warm, the edges of the moon a little bright. When darkness falls, Selin stays a jet-black silhouette in front of the moon.

When the moon has climbed, they shoot with the Looney rule, at f/11 and ISO 200 at 1/200 second: the dark patches on the surface are detailed but flat. A week later, when the moon reaches its last quarter, with the same lens the long shadows of the craters stand out along the terminator. The same night they turn to Jupiter: a bright speck and beside it four small points on a line.

Common mistakes

Summary

The moon comes out as a white blob because it is a surface lit by the sun and a long exposure set for the night sky burns it; the moon is exposed like daylight, at full moon at f/11 and a time equal to the inverse of the ISO, and the exposure is raised as the phase shrinks. Craters become distinct along the terminator, in the crescent and quarter phases; because the moon's angular diameter is half a degree, its size on the sensor is a hundred-and-tenth of the focal length. The moon is balanced with the landscape at twilight, and a combination is told; planets are small points, and the person in front of the moon is shown either as a silhouette or in their own tone, metered for their face at twilight.

Mini assignment

In this assignment you will photograph the moon and a planet.

1. Photograph the moon with the Looney rule, at f/11 and a time equal to the inverse of the ISO; look at the histogram and correct the exposure if needed.
2. Photograph the moon in a crescent or quarter phase and at full moon; compare the craters along the terminator, and calculate the moon's size on the sensor at the focal length you used.
3. Photograph the moonrise at twilight together with a landscape or a person; decide whether you will show the person as a silhouette or with their face readable.

Check yourself

• Do the moon's brightest regions carry detail without burning out?
• In which phase are the craters more embossed?
• Is the skin of the person at twilight in its own tone?

Glossary

Mini quiz

1. Why is the moon exposed like daylight?

• A) Because it produces its own light
• B) Because it is a surface lit by the sun
• C) Because the night sky isn't dark
• D) Because the camera wants it so

2. By the Looney rule, what is the starting exposure for the full moon at ISO 100?

• A) f/2.8, 30 seconds
• B) f/11, 1/100 second
• C) f/16, 1 second
• D) f/4, 1/15 second

3. When do the moon's craters look most distinct?

• A) At full moon
• B) When the moon is very close to the horizon
• C) In a crescent or quarter phase, along the terminator
• D) At new moon

4. With a six-hundred-millimeter lens, roughly how large is the moon's image on the sensor?

• A) Five and a half millimeters
• B) Thirty-six millimeters
• C) One centimeter
• D) Half a millimeter

Answers

1. B — The moon's surface is like a rocky place standing under the sun and reflects light at daylight brightness.
2. B — The aperture is f/11 and the time is the inverse of the ISO, that is, a hundredth of a second.
3. C — Slanting sunlight lengthens the shadows near the terminator and embosses the surface.
4. A — The moon's size is roughly a hundred-and-tenth of the focal length, and a hundred-and-tenth of six hundred is around five and a half.

Summary card

• The moon is lit by the sun: expose it like daylight.
• Looney rule: f/11, time the inverse of the ISO; raise the exposure as the phase shrinks.
• Craters stand out at the terminator, in crescent and quarter.
• The moon is half a degree: on the sensor, about a hundred-and-tenth of the focal length.
• Balance the landscape at twilight; show the person as a silhouette or metered for their face.

Bridge to the next lesson

Having gathered the frames of all the track's nights in one folder, your friend opens the Milky Way frames on the screen and is still not happy with something: the sky is noisy and there are colored specks in the dark regions. Someone at the club shows them an image in which ten frames shot from the same framing are combined into a single frame: the sky smooth, the dust lanes clear. How do you get a noise-free night sky? The next lesson takes up frame stacking, noise reduction and editing a night sky frame.