Multiple exposure

Learning goal

You will be able to combine several frames into a single photo inside the camera during the shoot, not afterward in a program. You will explain how the combining modes the camera offers, additive, average, lighten and darken, bring the frames' brightness together. In additive mode you will calculate how much underexposure to give each frame according to the number of frames, and grasp why the second image appears in the dark areas of the first frame and disappears in its bright areas. You will align the second frame by seeing the first frame overlaid on the screen, and plan placing a texture inside a silhouette. In a portrait you will decide which areas of the face the second image will enter by looking at the skin's brightness, and keep in mind that a combined frame shouldn't be presented as a documentary photo.

The question

The full moon is rising in front of the window. Your friend from the club wants to shoot their own face from the side as a silhouette against the window's bright sky, and see the moon's cratered disc inside this black shape. The first way that comes to mind is shooting the two photos separately and stacking them as layers in a program. Then, while digging through the menus, they come across an option that stacks frames at the moment of the shutter, and decide to try it.

In the first attempt the moon falls on the bright part of the sky and can barely be seen; in the second attempt the whole frame is too bright. After a few settings the moon appears right inside the darkness of the profile, like a texture bounded by the line of the forehead and the nose. How do two photos combine in one frame? Why does the moon disappear over the sky but show inside the face?

Context

In the compositing lesson we discussed combining parts of separate photos with masks in an editing program, and telling the viewer that such an image is a composite (Lesson 21.11). In the light direction lesson, too, you saw that a silhouette is the dark outer shape of a subject exposed for the background in backlight (Lesson 5.2). Multiple exposure brings these two together at the moment of the shoot: the silhouette's darkness becomes a space where the second image will settle.

The most important difference between multiple exposure and combining in an editing program is timing. The decision is made during the shoot; the two frames' positions relative to each other, their brightness and their composition are built at that moment. This is both a constraint and a discipline: there is no chance to change everything later, but you see the image you have while you are still in the scene, and correct it.

Simple explanation

How does the camera combine the frames?

Multiple exposure is combining several frames into a single photo inside the camera, during the shoot. In film cameras this was done by letting light fall on the same frame a second time without advancing the film; today's cameras read the frames from the sensor separately and calculate them with a combining mode. The names of the modes vary from camera to camera, but most fit one of four basic behaviors.

Additive mode is the mode that combines frames by adding their brightness at each point; this is the behavior most like exposing the same frame of film twice. If each frame is exposed normally, the total comes out too bright. If two frames will be combined, each is given one stop of underexposure, because two half lights make one full light. For three frames the underexposure is about one and a half stops, and for four frames two stops. Average mode is the mode that takes the average of the frames' brightness; the camera calculates this underexposure itself, and the total brightness stays close to that of a single frame.

The lighten and darken modes are the modes that keep, at each point, only the lighter or only the darker of the frames. In lighten mode, a moon in front of a dark background is added to the first frame without changing the background's darkness; in darken mode, a dark branch in front of a white background enters the image without affecting the background. Because these two modes don't add brightness, they don't need underexposure.

Where does the second image settle?

In additive and lighten modes, the core of the rule is this: the second image shows in the dark areas of the first frame and disappears in its bright areas. A bright sky is already bright; when the moon is added on top, that point becomes a little brighter or doesn't change at all, and the moon can't be made out. The dark profile of the silhouette, on the other hand, is like an empty surface; when the moon falls there, it appears in all its detail.

That is why, when a multiple exposure is planned, the first frame is set up to make room for the second image. In a silhouette portrait the profile is kept dark and the background bright; so that the second image shows only inside the profile, the second image is also shot in front of a dark background. The moon meets this condition by itself thanks to the night sky: its surroundings are black, and when added, only the moon itself is added to the first frame. Tree branches, a city's lights or flowers are also often used as the second frame; in each, which areas of the second image are bright decides what will show in the combination.

In darken mode the rule is reversed. When dark branches shot against a cloudy white sky are added with darken mode on top of a light-toned portrait shot in front of a white background, the branches appear in the light areas of the face, and disappear in the hair, which is already dark.

How do you align the frames?

Overlay display is the first frame being shown semi-transparently on the screen or in the viewfinder during a multiple exposure, and the second frame being framed according to it. Many cameras have this feature, and it takes the job out of guesswork: you see where the moon will fall inside the profile, and which line of the forehead the branches will pass through, before shooting. Some cameras also allow the first frame to be chosen from a photo saved on the card; this way the silhouette can be shot one day and the moon another night.

Focus and scale are planned too. The moon is a distant subject and needs to be enlarged with a long focal length; the silhouette, on the other hand, is close. The two frames can be shot with different lenses and different focus settings, because the camera exposes each frame separately. Many cameras also save each frame as a separate raw file alongside the combination; a program can later rebuild the same combination with a different proportion. Although this is a safeguard, the real value of multiple exposure is making the decision in the scene.

Why do the face and honesty need a separate decision?

In a portrait, when the second image enters the face, the skin's brightness decides what will show. In additive mode, dark skin acts as a dark area next to a bright background, and the texture of the second image can appear not only at the edges of the silhouette but also over the cheeks and forehead. Sometimes this is the desired effect; if it isn't, the face is exposed brighter in the first frame, or the bright areas of the second image are placed away from the face. Light skin, on the other hand, easily slips to white when added with the bright areas of the second image, and the face looks pale and faint; the second frame staying dark in that area, or average mode, prevents this fading. Which effect to choose depends on the photo's intention, but it is possible to see which one it is in the overlay display before shooting.

Although a multiple exposure is made at the moment of the shoot, the resulting image is a composite. The moon inside a silhouette, as an artistic expression, misleads no one. But adding another day's sky over a crowd in a news photo doesn't become honest because the shot was made inside the camera; when such an image is presented, it is stated that it has been combined.

Visual evidence

In the evening you set up the tripod in front of the window. Your friend sits side-on to the window; you meter the exposure for the bright sky and the profile comes out as a pitch-black silhouette. In the camera's multiple exposure menu you choose two frames and lighten mode and shoot the first frame.

You switch to the long focal length lens and turn to the moon. On the screen the silhouette shows semi-transparent; you bring the moon inside the profile, level with the temple and the eye. In the first attempt the moon has spilled over the tip of the nose, half of it lost over the bright sky. You shift the framing a little and shoot again. The moon's craters are now sharp in the darkness of the forehead and cheek; not a single piece spills into the sky, and the sky's brightness stays as it is.

Your friend wants to try an effect similar to the silhouette's dark profile on another friend's bright face too. In additive mode, when the bright areas of the second frame fall on the cheek, the light skin fades; when you realign the second frame so that its bright areas fall on the hair, the face's warm pink stays in place.

Common mistakes

Summary

Multiple exposure combines frames inside the camera during the shoot: in additive mode, because brightness is added, two frames are each given one stop of underexposure, average mode calculates this itself, and lighten and darken modes keep only the lighter or darker one at each point. In additive and lighten modes, because the second image appears in the dark areas of the first frame and disappears in its bright areas, a silhouette is a ready space for the second image, and the frames are aligned with the overlay display. In a portrait, the skin's brightness decides where the second image will show on the face; although it was made at the moment of the shoot, a combined frame is a composite and isn't presented as a documentary photo.

Mini assignment

In this assignment you will combine two frames during the shoot with the camera's multiple exposure mode.

1. Write about how many stops of underexposure you need to give each frame if two, three and four frames will be combined in additive mode.
2. Shoot a silhouette against a bright window or sky; as the second frame, add a subject with a dark background, the moon, city lights or a flower in front of black paper, aligning it inside the profile with the overlay display.
3. Place the same second frame once in the bright area outside the silhouette and compare the two results.
4. In a portrait, note how the skin is affected when you place the second image on the face; if needed, change the alignment or the combining mode so that the face stays in the tone you want.

Check yourself

• Can you explain the difference between additive and average mode and the underexposure calculation?
• Can you say why the second image appears in dark areas and disappears in bright areas?
• In a portrait, did you notice how the second image changed the face's tone, and decide accordingly?

Glossary

Mini quiz

1. If two frames will be combined in additive mode, how much underexposure is given to each frame?

• A) None
• B) One stop
• C) Two stops
• D) Three stops

2. In lighten mode, the moon falls on the bright sky of a silhouette portrait. What happens?

• A) The moon looks more pronounced
• B) The sky darkens
• C) The moon almost disappears into the brightness
• D) The silhouette brightens

3. What is the most reliable way to place the second frame in the right spot relative to the first frame?

• A) Seeing the first frame on the screen with the overlay display and framing according to it
• B) Shooting the frames with the same lens
• C) Choosing average mode
• D) Overexposing the second frame

4. In additive mode, the bright areas of the second frame fall on the cheek of a light-skinned face and the face fades. Which is the most suitable solution?

• A) Shooting the first frame brighter
• B) Adding a third frame
• C) Choosing darken mode and overexposing the second frame
• D) Realigning the second frame so that its bright areas fall away from the face

Answers

1. B — Two half lights make one full light; one stop of underexposure on each frame brings the total to normal.
2. C — Lighten mode keeps the lighter one at each point; the moon can't be made out in a sky that is already bright.
3. A — The overlay display shows where the second image will fall before shooting.
4. D — When brightness is added, light skin slips to white; moving the second frame's bright areas away from the face preserves the tone.

Summary card

• In additive mode, two frames = one stop under on each; average mode does this itself.
• Lighten mode keeps the light, darken mode keeps the dark.
• The second image appears in darkness and disappears in brightness; a silhouette is a space.
• Align the frames with the overlay display.
• Check the skin's tone on the face; don't present a combined frame as a document.

Bridge to the next lesson

The next morning your friend stopped in front of the row of poplars on the bank of the stream. They described a photo they had seen at an exhibition: the tree trunks had turned into vertical brushstrokes instead of sharp lines, and the leaves had flowed as patches of color. The photographer had deliberately pulled the camera upward while the shutter was open. Your friend wasn't sure whether this was a mistake or an expression. Can shaking the camera be art? The next lesson tells about intentional camera movement, which creates a painterly effect with controlled movement of the camera at a slow shutter speed.