Exposure lock and bracketing

Learning goal

You will learn to find the right exposure once in difficult light and keep it, and to take out insurance when you can't find it. With exposure lock you will meter a point, lock it, and set up the frame again without breaking the lock; you will tell the lock apart from manual mode and from focus lock; with auto exposure bracketing you will record a single moment at several brightnesses; you will choose the bracketing step and the number of frames for the scene; you will use bracketed frames for choosing or for merging; and you will make sure faces, on different skins, are recorded correctly in at least one frame.

The question

At a wedding, the moment the bride walks into the hall; on a street, the moment a child throws bread to the birds; on a hilltop, the moment the sun comes out between the clouds for an instant: these moments happen once and don't come back. When the light is difficult, there is often no time to hit the setting by trial and error. You take a frame, look at the screen, and the moment has long gone.

Is it possible not to leave things to luck in difficult light? It is. The tools for this are exposure lock, which holds the right reading in place once you find it, and bracketing, which records the same scene at several separate brightnesses and leaves you options when you can't find it. In this lesson we will talk about these two safeguards and when to turn to which one in difficult light.

Context

In Lesson 4.3 you learned to lock focus and exposure on a phone by tapping the screen and keeping your finger pressed. On cameras the same job is usually done with a separate button, and combined with spot metering it turns into a very powerful tool. In this module you saw how the meter goes wrong, metering modes, compensation and dynamic range. Exposure lock lets you use all of this knowledge in a single move: you meter the right place, lock the result, and set up the frame as you like.

Bracketing works with a different logic. Instead of finding the right exposure, it takes several of the possible exposures at once. One frame is at the camera's suggestion, one darker, one brighter. Later you choose the best one, or merge them as in the dynamic range lesson and widen the range. The lock is a tool for deciding, bracketing is an insurance policy. Together, they are ways of not leaving difficult light to luck.

Simple explanation

How does exposure lock work?

Exposure lock freezes the exposure the camera is measuring at that moment. From the moment you lock it, even if you change the frame or turn the camera in another direction, the exposure doesn't change. Many cameras have a separate button for this, called auto exposure lock; it is usually marked with the letters AE-L or a small star icon. On some cameras the lock lasts as long as you keep the button pressed; on others it locks with one press and unlocks with a second press; this behavior can be changed in the menu.

The lock's most powerful use is a three-step move: meter, lock, recompose. First you bring the point you want to meter into the middle of the frame or under the spot metering point. Then you press the lock. Finally you set up the frame as you like and shoot. That way, even if the subject stands at the edge of the frame, its exposure comes from the reading in the middle. Exposure lock and focus lock are separate things; on many cameras they work with separate buttons or settings. Half-pressing the shutter button locks focus and exposure together on some cameras, and only focus on others; you need to learn your own camera's behavior from the manual.

When is the lock better than manual mode?

Exposure lock and manual mode do a similar job: both keep the exposure fixed. The difference between them is duration. In manual mode the settings are fixed until you change them; this is ideal when you work in the same light throughout a scene. Exposure lock, on the other hand, is valid for a single frame or a short series; when you let go of the lock, the camera starts measuring again. That is why the lock is more practical for situations where the light keeps changing but you want to catch a single moment right.

For example, spot-metering a face in backlight and locking it, then placing the person at one edge of the frame, is a job of a few seconds. Metering a gray rock in the sun on a snowy slope and locking it, then turning the frame to the snow, works the same way. The lock's trap is this: if you keep shooting with a locked exposure for a long time, the photos drift when the light changes. That is why in every new scene, or when the light changes, you need to meter and lock again. The lock is single-use; manual mode is a working setting.

How do you set up bracketing?

Auto exposure bracketing takes the same scene several times at different exposures when you press the shutter button once or several times in a row. Usually three or five frames are chosen: one at the camera's suggestion, the others below and above it. The difference between the frames, that is the bracketing step, can be set on most cameras from a third of a stop up to several stops. Small steps, for example a third or two-thirds of a stop, are for making a fine choice around the right exposure; large steps, for example two stops, are for frames that will be merged to widen the dynamic range.

Using bracketing together with continuous shooting shortens the time between frames. If the subject is moving, small differences appear between the frames; this can make merging harder later and leave ghost traces in the merged frame. When bracketing for merging, placing the camera on a tripod or a steady surface gives the best result. If it is only for choosing the best frame, shooting handheld is usually enough. Don't forget to turn bracketing off after using it; otherwise in later shoots it keeps taking several frames for every moment.

How do you use bracketed frames?

The first use of bracketed frames is choosing: from the differently exposed versions of the same moment you choose the best and delete the others. This is insurance for situations where you couldn't find the right exposure at the moment of shooting; each extra frame is a safety frame. A more advanced use is merging: you merge the frame exposed for the sky with the frame exposed for the shadows in an editing program, and gather into one photo a range a single frame can't record.

Faces need separate attention here too. In each bracketed frame the face is recorded in a different tone: in the dark frame the shadows of dark skin can be lost, in the bright frame the forehead of light skin can blow out. When choosing, don't look only at the histogram; zoom in on the face in each frame. If there are people of different skin tones in the same frame, the frame where each of them looks best is usually different; here you either choose the middle frame where both look reasonable, or narrow the difference in the next shot by changing the light. In merged frames, check separately that faces look natural, not flat or gray.

Visual evidence

Imagine photographing an elderly coppersmith working on the threshold of an arched doorway in the courtyard of a historic inn. The coppersmith is in the shade, their dark-skinned face bent over the copper tray they are hammering; behind them are the sunlit courtyard and a bright sky. You want to place them at the left edge of the frame and the courtyard on the right. The frame you take with matrix metering has exposed the courtyard correctly, but left the coppersmith as a dark shape.

You switch to spot metering, bring the coppersmith's shaded cheek into the middle of the frame, meter, set compensation to a third of a stop minus, and press the exposure lock. Then, without letting go of the lock, you turn the camera, place the coppersmith at the left edge and the courtyard on the right, and shoot. The coppersmith's face, the warm depth of their skin and the gleam of the tray can be made out; the courtyard, on the other hand, is blown out. Because you also want to protect the architecture, you lean the camera against a column and bracket three frames at a two-stop step. In the dark frame the courtyard's stones and the sky can be made out; in the bright frame the coppersmith's shadow is fully open. In the evening you merge two frames, and in the merged frame you zoom in on the coppersmith's face and check that their skin hasn't slid toward gray.

Common mistakes

Summary

Exposure lock lets you meter the right place, freeze the result and set up the frame again without breaking the lock: meter, lock, recompose; it is single-use and should be renewed when the light changes, while manual mode is a fixed setting for a whole scene. Auto exposure bracketing takes the same moment at different exposures and offers insurance: with small steps, for choosing the best frame, and with large steps, for merging frames and widening the dynamic range. In every locked or bracketed frame you need to zoom in on the face separately, and make sure the shadows and bright points of different skins are recorded correctly in at least one frame.

Mini assignment

This assignment is the module project: "White on white" and "black on black".

1. Prepare a white object on a white surface, for example a white cup on a white tablecloth, and a black object on a black surface.
2. Put a gray card next to the two setups in the same light, spot-meter it, lock the exposure, set up the frame and shoot.
3. Shoot the same two scenes again, bracketing three or five frames at a step of a third of a stop, and, looking at the histogram, choose the frame that shows white as white and black as black.
4. Add the two frames you chose to your portfolio folder, and under each one write which reading, which lock or which bracketed frame you used.

Check yourself

• In the white-on-white frame, are the whites not gray, while their textures can still be made out?
• In the black-on-black frame, are the blacks black, while their shapes can still be told apart?
• Can you explain why you chose the frame from the bracketed series, relying on the histogram and your eye?

Glossary

Mini quiz

1. What is the basic function of exposure lock?

• A) Fixing the focus
• B) Freezing the measured exposure and keeping it even if the frame changes
• C) Raising ISO automatically
• D) Locking the white balance

2. What is the basic difference between exposure lock and manual mode?

• A) The lock is valid for a single moment or a short series, while manual mode stays fixed until you change it
• B) Manual mode is only found on phones
• C) The lock only works at night
• D) There is no difference between them

3. When bracketing frames that will be merged to widen the dynamic range, which step is more suitable?

• A) Zero stops
• B) Only a third of a stop
• C) The step doesn't matter
• D) A large step such as two stops

4. When choosing one of the bracketed frames, if there are two people of different skin tones, what do you do?

• A) Choose the frame with the largest file size
• B) Choose the darkest frame
• C) Zoom in on both faces in each frame and choose the frame where both look reasonable
• D) Choose the brightest frame

Answers

1. B — The lock freezes the exposure at the moment of metering; even if the frame changes, the exposure stays the same and the subject is correctly exposed at the edge too.
2. A — The lock is a single-use decision for the light at that moment; manual mode is a setting that stays fixed throughout a whole scene.
3. D — In frames to be merged you need to catch the ends; large steps protect the sky and the shadows in separate frames.
4. C — One skin's best frame may not be the other's; zooming in on both and looking gives a balanced choice.

Summary card

• Exposure lock: meter, lock, recompose.
• The lock is single-use; when the light changes, meter again.
• Bracketing: one moment, several brightnesses.
• Small step for choosing, large step for merging.
• Zoom in on the face in every frame; turn bracketing off when you are done.

Bridge to the next lesson

In this module you learned to control exposure, that is, how bright the photo will be. But a correctly exposed photo is still flawed if the sharpness is in the wrong place. The next module moves on to sharpness, and its first question is this: how does the camera know what to focus on? There we will talk about how autofocus sees the scene with two different methods.