Flash and ambient light

Learning goal

You will see that every frame using flash is actually made of two separate exposures recorded at the same time, and you will be able to set these two independently of each other. You will explain that ambient exposure depends on shutter speed, aperture and ISO, while flash exposure depends on aperture, ISO, flash power and distance, and that because the flash is a very short burst, below the sync speed it isn't affected by shutter speed. You will set the brightness of the background with the shutter speed and the brightness of the subject with the flash power, build the ambient-to-flash ratio between the two as an expressive decision, bring back a dark background with slow sync, and when the colors of the two lights diverge, check this on the face and the background separately. You will also take into account how the edges of different skin tones can be lost in front of a dark background.

The question

The evening sun has just set; you are on a small fishing pier with your uncle. You want to shoot a portrait of your uncle as he packs up his fishing rod. Behind him there is a dark blue sky and city lights that have started to come on across the water. When you turn on the flash and shoot, his face is properly lit but the back is pitch black: the sky, the shore, the lights have vanished, and your uncle stands alone in a dark void. You slow the shutter speed and shoot again. Your uncle's face hardly changes at all, but behind him the blue of the sky and the lights of the shore come back. When you stop down the aperture, this time both the face and the sky go dark together.

Why are there two exposures in one photo? Why does the shutter speed change only the background and not the face, and why does the aperture touch both at once?

Context

While talking about who should decide the flash's power (Lesson 19.3), we always looked at the subject itself: is the face at the right brightness? But in a flash frame there is also a world behind the subject, and that world is lit not by the flash, but by the setting's own light. The sky after sunset, the chandeliers of a hall, the city lights are too far for the flash to reach.

That is why setting up a flash photo requires thinking of two separate light sources as two separate exposures. A photographer who knows which settings these two exposures depend on can light the subject and the background independently of each other. By the end of this lesson, the flash will no longer be a light that crushes the scene, but a tool that blends with the ambient light.

Simple explanation

Where do the two exposures come from?

Ambient exposure is the record in the frame of the light already present in the scene, not the flash's, that is, the sky, the lamps, the windows. This light keeps flowing onto the sensor as long as the shutter stays open; that is why it is affected by shutter speed, aperture and ISO. Halving the shutter speed raises the ambient light by one stop, just as in a shot without flash.

Flash exposure, on the other hand, is the part of the frame lit by light coming from the flash. The flash fires and goes out in a very short moment while the shutter is open. This burst lasts much shorter than all the shutter speeds below the sync speed; no matter how long the shutter stays open, all of the flash's light reaches the sensor in that short moment. That is why changing the shutter speed hardly changes the flash light at all. Flash exposure is decided instead by the aperture, the ISO, the flash's power and the flash's distance from the subject.

When you put these two lists side by side, the result becomes clear. The shutter speed changes only the ambient light, the flash power and distance change only the flash light. The aperture and ISO, on the other hand, change both together. In your uncle's portrait, when you slowed the shutter speed only the sky brightened; when you stopped down the aperture, both the face and the sky went dark.

The sync speed is the shortest shutter speed at which the flash can light the whole frame evenly. In most cameras the shutter has two curtains, and at very short shutter speeds the whole sensor is never fully open at any moment; the curtains pass over the sensor as a narrow slit. If the flash fires in this situation, part of the frame stays in the curtain's shadow and a black band appears in the photo. The sync speed varies from camera to camera and is written in the camera's manual; in many cameras it is around a two-hundredth of a second. When working with flash, the shutter speed is generally kept below this value.

How are the two exposures set?

A practical order is this. First turn off the flash and set up an ambient exposure for the background only: decide how bright or dark you want the sky to look and choose the shutter speed accordingly. Then turn on the flash and set the brightness of the face with the flash power, or in TTL with flash exposure compensation. During this, touching the shutter speed changes the background, touching the flash power changes the face; the two don't spoil each other.

For example, if at ISO 400 and f/4, at a two-hundredth of a second, the sky comes out almost black, lowering the shutter speed to a fiftieth of a second brightens the background by two stops; the face stays the same. If you stop down the aperture from f/4 to f/5.6, both the background and the face go one stop darker; to keep the background where it was you need to slow the shutter speed by one stop, and to keep the face where it was you need to double the flash power.

The ambient-to-flash ratio is the difference in brightness between the flash light on the subject and the ambient light in the background. Keeping the background one or two stops darker than the subject lets the subject stand out and the scene keep its evening mood. Keeping the background at the same brightness as the subject almost hides the presence of the flash; the subject looks as if naturally lit by ambient light. Darkening the background completely gives a dramatic, studio-like look. None of these options is right on its own; which one is chosen depends on what the photo wants to say.

What does slow sync bring back, and how do the two colors meet?

Slow sync (dragging the shutter) is using a long shutter speed together with the flash to bring the light of a dark setting into the photo too. On a street or in a hall at night, firing the flash with a normal shutter speed darkens the back; lowering the shutter speed to a tenth of a second or slower brings back the lights behind. The price has to do with movement: the flash freezes the subject sharply, but while the shutter stays open, the ambient light can leave a slight trail, a ghost, around a moving subject. Sometimes this trail is used on purpose to tell movement; sometimes you need to keep the subject still or speed up the shutter a little.

The colors of the two lights are most often different. The flash's light is a neutral color close to daylight; bulbs and streetlamps are much warmer. When you set the white balance for the flash, the subject looks neutral and the lamps behind orange; this is the flash form of the deliberate color contrast we talked about in the mixed light lesson (Lesson 18.6). If the background's orange spills around the face or spoils the atmosphere you want, you need to bring the flash's color closer to the ambient light; we will come back to the tools for this at the end of the module.

A dark background causes different problems on different skin tones. When you keep the background dark, the edges of a dark-skinned person's hair, shoulders and chin can merge with the dark background, and the face looks as if hanging in a void; in this case, narrowing the ambient-to-flash ratio a little or putting a brighter part of the background behind brings the edges back. A light-skinned person, on the other hand, can look too bright and chalky with flash in front of a dark background; turning the flash power down a little softens the skin. In both cases you need to enlarge the face and check both the skin and the edges.

Visual evidence

With the settings you shot the first frame with on the pier, ISO 400, f/4 and a two-hundredth of a second, your uncle's dark skin is lit warm and lively by the flash, but the edge of his hair and shoulders has merged with the pitch-black sky behind him; his face hangs as if in a void. You turn off the flash, meter for the sky only, and lower the shutter speed to a fiftieth of a second: the sky turns a deep, dark blue, and the lights of the far shore appear as warm points.

You turn the flash back on. The face is at the same brightness, but now there is a living evening behind it. The line of your uncle's shoulders separates clearly against the blue sky, and the edge of his hair can be made out. The background is about one stop darker than the face; your uncle stands out, but the mood of the evening isn't lost. In a frame you shot while he was swinging the fishing line, there is a slight trail around the line; in the next frame you ask him to hold the line still for a moment.

Common mistakes

Summary

Every flash frame is the sum of two exposures: ambient exposure depends on shutter speed, aperture and ISO; flash exposure depends on aperture, ISO, flash power and distance, and because the flash is a very short burst, below the sync speed it isn't affected by the shutter speed. That is why the background is set with the shutter speed and the subject with the flash power, separately; the ambient-to-flash ratio between them is an expressive decision that decides how much the subject stands out and the mood of the scene. Slow sync brings back a dark background but leaves a trail on a moving subject, the different colors of the two lights are checked separately, and care is taken that the edges of the skin aren't lost in front of a dark background.

Mini assignment

Do the following tries at evening twilight or in a room with the lamps on.

1. Shoot a person with flash at a two-hundredth of a second; then, without touching the flash power, lower the shutter speed to a fiftieth and a fifteenth of a second and take two more frames. For each frame, write down side by side the brightness of the face and the tone of the sky.
2. Shoot the same person keeping the shutter speed fixed and stopping down the aperture by one stop; see both the face and the background go dark, then correct the shutter speed and the flash power separately to bring both back.
3. Shoot three frames with the background two stops darker than the face, one stop darker and at the same brightness, and decide which ratio suits the scene.
4. Raise the shutter speed one or two steps above the sync speed, shoot a frame with flash and find the black band.

Check yourself

• Did you see in your own frames that the face stayed almost the same when you changed the shutter speed?
• Looking at a scene, can you say with which settings you would manage the background and the subject separately?
• In front of a dark background, did you check whether the edges of the face and hair could be made out?

Glossary

Mini quiz

1. In a flash frame, what does slowing the shutter speed change?

• A) Only the background lit by ambient light
• B) Only the face lit by flash
• C) Both the face and the background equally
• D) Neither

2. When you stop down the aperture from f/4 to f/5.6, what should you do to bring the face and the background back to their old brightness?

• A) Only slow the shutter speed by one stop
• B) Only double the flash power
• C) Slow the shutter speed by one stop and double the flash power
• D) Change the white balance

3. What happens when you raise the shutter speed above the sync speed and shoot with flash?

• A) The flash fires more strongly
• B) A black band appears in part of the frame
• C) The background goes dark but the face doesn't change
• D) Red-eye disappears

4. When you shoot a dark-skinned person with flash in front of a dark background, the edges of their hair and shoulders merge with the background. Which is the most fitting solution?

• A) Set flash exposure compensation to plus
• B) Crop the photo afterward
• C) Lower the ISO
• D) Slow the shutter speed and narrow the ambient-to-flash ratio to brighten the background a little

Answers

1. A — The flash is a very short burst; the shutter speed changes only the continuously flowing ambient light.
2. C — The aperture cuts both exposures at once; the background is brought back with the shutter speed, the face with the flash power, separately.
3. B — Above the sync speed the curtains don't fully open the sensor, and part of it stays in shadow at the moment of the flash.
4. D — Brightening the background separates the edges from it; raising the flash power only blows out the face.

Summary card

• A flash frame is two exposures: ambient light and flash.
• The shutter speed changes only the ambient light, the flash power and distance only the flash.
• The aperture and ISO change both at once.
• Don't go above the sync speed; a black band appears.
• Set the background with the shutter speed, the subject with the flash; check edges and skin by enlarging.

Bridge to the next lesson

The evening portrait on the pier got easier as the sky darkened: slowing the shutter speed was enough. The next day at noon, when you want to fill the hard shadows on your uncle's face with flash on the same pier, you run into the opposite problem: to darken the sunny sky you need to speed up the shutter, but the moment you go above the sync speed, a black band appears in the frame. How do you use flash in bright sun? The next lesson takes up high-speed sync, which lets you use flash above the sync speed too, and its price.