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Digital cameras may boast ever-increasing resolutions but, like film cameras, they're capable of capturing only a limited range of tones from light to dark. This explains why many photographs rarely measure up to what we experience in person. We may see faint wispy clouds in bright skies, but our cameras often just capture a great expanse of white, while subtle details in dark shadows can be lost in a solid, featureless black.

The range of tones a camera can capture is known as its dynamic range. Think of it as a relatively short bar positioned on a longer line, which represents real-life tones with pitch black at one end and intense brightness at the other. By adjusting the camera's exposure, you can shift this small bar to the left or right extremes of the longer line to capture detail in dark shadows or bright highlight areas, or just leave it in the middle to record average mid-tones. Sadly, the limited dynamic range of digital cameras means you can't capture a complete range of tones in a single photograph. Or can you?

Photographers have long exploited exposure bracketing to capture multiple variations of the same scene. Shorter and longer exposures can reveal detail in bright and dark areas respectively, complementing the range of tones captured in a normal shot. By taking the best parts of each separate exposure, you could enjoy photographs that retain detail across a far wider tonal range. The trick is finding a way to assemble them convincingly.

Photographers used to painstakingly layer and retouch the best portions of multiple

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images by hand, but now several software utilities can perform the process automatically to generate a single image with a high dynamic range (HDR).HDR was first seen in Photoshop CS2, but is now supported in more affordable packages and is fast becoming a technique of choice for many.

HDR is well suited for landscapes or interiors, but can work wonders with almost any kind of shot. The software tools allow you to be subtle with your extended tonal range or produce outrageous effects. In this article, we'll explain everything you need to know.

Getting started

The first step in creating an HDR image is to take several different exposures of the same scene. The goal is to use a range of exposures that will capture all the tonal detail, from dark shadow to bright highlight. The number of images you take is up to you, based on the requirements of the scene itself, though most HDR packages recommend a minimum of three exposures.

For the software to produce a convincing result, each exposure must have a certain degree of overlap in the tonal range it is capturing. The general rule for HDR is to vary the exposure by one stop for every image, which means doubling or halving the exposure each time. The obvious way to achieve this is to use your camera's exposure compensation controls, but on most cameras these operate only over a limited range of plus or minus two stops, which may not be sufficient for an HDR image with an extreme range of tones. Also, the exposure compensation controls may adjust the exposure by changing the aperture, which is undesirable for HDR as the depth of field will change between images.

The best way of changing the exposure for HDR images is to adjust only the shutter speed. Put your camera into manual mode, set the aperture for the desired depth of field and adjust the shutter speed until your camera reports a correct 'normal' exposure. Take this photo, then double the shutter speed and take another. Repeat this, doubling the shutter speed each time until you have all the required exposures for the brightest areas. The last photograph you take will look very dark.