Life after Photoshop

How HDR works: exposure bracketing, merging, tone mapping and effects

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How HDR works
The screenshots used here are from Aurora HDR 2019. Other HDR software is available.

HDR is a technique for capturing scenes with a very high brightness (dynamic) range. This is measured in f-stops or EV values, and digital camera sensors can typically capture up to 12 stops of dynamic range.

Read more: Aurora HDR 2019 review

Much of the time your camera’s 12-stop dynamic range ‘window’ is much wider than the scene you’re photographing, but sometimes the scene’s dynamic range is wider than your sensor’s and if you need to maintain detail in the highlights and shadows, this is when you need HDR techniques.

How HDR works
This shot is a good candidate for HDR, since there’s no single exposure which will be ab le to capture detail in the darkest shadows of this building’s wooden rafters and the bright sky outside.

HDR exposure bracketing

The usual HDR technique is to take a series of the scene at different exposures and merge them using HDR software. There are a number of dedicated HDR plug-ins and apps on the market and some image editors offer HDR merging tools built in. They all work on broadly the same principle (Skylum Aurora HDR is being used for these examples).

HDR merging

HDR tools usually offer three additional options for the merging process. First, image alignment. If you used a tripod you won’t need this, but if you shot your exposure series handheld, you will need to rely on the software to line up the shots precisely.

How HDR works
The classic HDR technique is to shoot the scene using a series of different exposures and merge them using HDR software.

The ghosting option is to take care of any movement in the scene between exposures. You might have pedestrians walking in front of the camera or leaves blowing in the wind. The software will typically select a single frame as a reference and suppress or hide any movement in the other frames.

How HDR works
HDR merge tools will typically offer optional auto alignment, ghost reduction and chromatic aberration reduction as part of the merge process.

There will often be a chromatic aberration removal option too. HDR techniques can exaggerate colour fringes around object edges, so removing them at this stage is a good idea.

HDR tone mapping and editing

This process of merging separate exposures into a single image is sometimes called ‘tone mapping’. Merged, or tone-mapped, images will typically be in an extended 32-bit colour space where you use editing tools to enhance the image and HDR effect.

How HDR works
The HDR tone mapping process can make the whole brightness range in the scene visible, but it may look pretty flat without further editing.
How HDR works
This is why HDR programs typically come with a selection of preset effects. These consist of structure, clarity, saturation, curves, shadow and highlight adjustments you can also configure yourself.

Many programs come with preset image effects that give you a great result with a single click or make a good starting point for your own adjustments.

Once you’re happy with your image, you can then export it as a JPEG or TIFF image that can be shared with other people or opened/edited in other software.

HDR from single RAW files

Sometimes the camera sensor can capture the full brightness range in a scene with a single exposure, but you still need the tone mapping abilities of HDR software to produce an effective and dramatic-looking image.

For this, you should be shooting RAW files rather than JPEGs. Typical consumer cameras can capture only about 10 stops of dynamic range in JPEGs compared to the 12 stops in RAW files.

Most HDR tools will work with single RAW images as well as bracketed exposures, and will offer the same HDR enhancement options and presets – and you won’t get any problems with image alignment or ghosting.

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