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Technique3 min read

LESSON 57

HDR and bracketing

Several exposures for one wide range.

IN THIS LESSON

  1. The problem of dynamic range
  2. An old idea
  3. Bracketing and tone mapping
  4. In practice: when and how

The problem of dynamic range

Every sensor and film records only a limited range of brightness in one exposure. Beyond it, bright tones burn out to pure white and dark tones crush to pure black. The ratio between the brightest and darkest tones a single image can hold is its dynamic range, measured in stops. Many real scenes, such as a room with a sunny window, or a landscape with a bright sky and deep shade, hold more than that. The eye copes because it responds to light roughly logarithmically and adapts all the time. Multi-exposure HDR capture combines several differently exposed frames into one image that holds a wider range.

An old idea

Combining exposures is not new. In the 1850s Gustave Le Gray made seascapes using one negative for the bright sky and another, exposed longer, for the darker sea, then combined them in a single print. In the darkroom, dodging and burning later did a similar job by hand, giving more or less exposure to parts of a print. Today many phones capture and merge several frames automatically when they detect a high-contrast scene. On a camera you usually do it yourself.

Bracketing and tone mapping

The capture is called exposure bracketing: a series of frames of the same scene at different exposures, for example one normal, one darker and one brighter. Any camera with manual exposure can do it, and automatic exposure bracketing makes it quicker. Software merges the frames into one HDR image, which then has to be tone mapped, because screens and paper cannot show that range. Tone mapping compresses the range back down while keeping local contrast. Done gently, the result looks natural; pushed hard, it gives the grey skies and glowing edges many people associate with HDR.

In practice: when and how

This section is working practice. Try a single RAW frame first; modern sensors often hold enough range, and lifting shadows in the edit is simpler. When the histogram is clipped at both ends, bracket. Use a tripod, aperture priority so depth of field stays fixed, and steps of one to two stops. Moving things, such as leaves, people and water, cause ghosts because the frames do not match; the source notes this trade-off between range and sharpness. Most merge tools can suppress ghosts by taking that area from one frame. Keep the tone mapping light, and judge it the next day.

Try it today

  1. 01Find a room with a bright window and a darker interior.
  2. 02Set aperture priority, a tripod and three-frame bracketing at two-stop steps.
  3. 03Shoot the set, and a single RAW frame for comparison.
  4. 04Merge the set and tone map it gently.
  5. 05Compare it with the single frame edited by lifting shadows and lowering highlights.