Have you ever photographed a scene with a large difference between the bright and dark areas? You may find yourself choosing between preserving detail in the highlights and retaining information in the shadows. Sometimes there is no single exposure that captures the full range of tones you want.
One solution is to take a series of photographs at different exposure levels. Doing this manually takes time, particularly when the light is changing quickly. Most cameras offer Automatic Exposure Bracketing (AEB), which automates much of the process.
With exposure bracketing, the camera takes a sequence of frames at different exposure values around the metered exposure. Depending on the camera, you can choose the number of frames and the exposure increment. The result may be a normally exposed frame, one or more underexposed frames and one or more overexposed frames.
The frames can be blended to create a high-dynamic-range (HDR) image. They can also be used individually, giving you more choice when deciding how to process the scene. Bracketing does not automatically create a better photograph; it simply gives you more exposure information to work with.
Aperture priority or shutter priority?
The way AEB changes the exposure depends on the camera and the exposure mode. In aperture-priority mode, many cameras keep the aperture fixed and vary the shutter speed. In shutter-priority mode, they commonly keep the shutter speed fixed and vary the aperture. Check your camera’s manual, as behaviour differs between models.
Changing aperture also changes depth of field, which may be undesirable if you intend to merge the frames. For that reason, I often prefer to keep aperture fixed and let the camera vary shutter speed. In manual exposure mode, some cameras also vary shutter speed during bracketing, but this is not universal.
Exposure is controlled through aperture, shutter speed and ISO. Which parameter the camera changes during AEB depends on the selected mode and the camera’s implementation. Some cameras allow you to specify this behaviour; others do not.
On my Sony Alpha A99V, the behaviour depended on the exposure mode. In aperture-priority mode, the camera varied shutter speed; in shutter-priority mode, it varied aperture. In program and manual modes, the behaviour was different. These are observations about my A99V, not a general rule for other cameras.
There is an important limitation: if the camera reaches the end of its available shutter-speed or aperture range, it may be unable to apply the full exposure increment. Check the resulting frames rather than assuming every bracketed exposure is exactly as requested.
What about flash?
Flash bracketing is not necessarily the same as ambient-light exposure bracketing. Depending on the camera and flash system, the sequence may vary flash output while keeping the camera’s aperture, shutter speed and ISO unchanged. Some systems handle flash bracketing differently, so check the instructions for your equipment.
Keep the camera steady
You can bracket handheld, but a tripod makes it easier to align the frames when you merge them later. Modern software can correct small shifts, but it cannot always compensate for movement within the scene. Leaves, water, people and clouds may change between exposures and create ghosting in the final HDR image.
Lock focus
For a sequence intended for HDR merging, keep focus consistent. Use manual focus or lock autofocus after focusing, so the camera does not choose a different focus point for each frame. This is particularly important when depth of field is shallow.
When AEB is not ideal
Bracketing is less useful with fast-moving subjects, such as aircraft, racing cars or birds in flight, because the subject may move between frames. It can also create problems when the scene itself changes quickly, or when changing aperture alters the depth of field. For moving subjects, a single RAW exposure may be the better choice.
Combining the images
Adobe Camera Raw includes an HDR merge function, and other applications offer similar tools. The software aligns the images and combines the tonal information. You can then adjust exposure, highlights, shadows, contrast and colour to suit the photograph.
Camera Raw is part of Adobe’s photography software workflow rather than a universally free standalone tool. The software available to you will depend on your Adobe plan and version.
The aim is not to make every part of the photograph equally bright. Use the extra information to create a balanced image while keeping the light and atmosphere that made the scene interesting.
Historical note: The camera examples in this article are based on my Sony A99V. AEB options and menu settings vary considerably between camera models.
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