With iOS 4.1, Apple added an HDR (high dynamic range) mode to the iPhone 4’s back-facing camera. When turned on, the mode quickly snaps three images at different exposures and then combines them to create an image that shows more detail in both the shadows and highlights.

It’s tempting to leave this setting turned on all the time. After all, you can set the camera to save a regular copy of each photo to your Camera Roll, along with the HDR version (Settings > Photos). Unfortunately, each HDR photo takes about five seconds to save. And if you’re short on memory, saving an additional, larger version of each image file adds up fast.

However, you can solve your time and space issues by learning which situations call for the iPhone’s HDR setting and which don’t.

When to use HDR
This setting can do wonders for many common iPhone photo situations.

Landscapes If a landscape shot has bright sky above the horizon line and darker foreground below, the HDR mode will combine the best of both areas. It also works great on night-time cityscapes (see image below). However, the mode falls short when used on shots of sunsets. By attempting to tone down the overexposed sun, the HDR setting removes some of the sunset’s beautiful red and orange hues, negating the powerful impact that the setting sun has.

Outdoor portraits The sun’s harsh light can make for unflattering portraits. It casts strong shadows on a person’s face and bounces off skin to accentuate shiny spots. Use the iPhone’s HDR mode to minimise these extremes and create an evenly lit portrait.

If a subject is completely backlit, tap to focus on the darkest part of the person’s face. After you take the shot, the final HDR photo will combine the properly exposed person with a slightly toned-down background.

Editing with apps If you plan to use an app to edit your image after you take it, an HDR shot will contain more information to work with. If you like the even exposure of an HDR photo, but are disappointed with its dull colouring, you can increase the saturation in a fully featured editing app such as Photogene or Adobe Photoshop Express.

A picture of a night-time city skyline (left) can be greatly improved by using the iPhone 4’s HDR feature (right)

When not to use HDR
Even though HDR can improve many photos taken on the iPhone, that doesn’t mean it’s ideal for every situation. Here are some occasions when it’s best to turn off the feature.

Capturing motion Because the iPhone camera takes three photos in quick succession when in HDR mode, it isn’t great for capturing motion. If the subject is fast moving, or if you move the iPhone while shooting, the final HDR image will show ghosting – that’s when the images aren’t aligned and objects appear in multiple places.

When contrast is key A good photo may create a sense of drama by contrasting light and dark; for example, an image might play up the impact of a strong shadow, or of a completely black silhouette against a bright background. HDR shots will decrease an image’s contrast, diminishing its impact.

Capturing vivid colours HDR can bring colours back into blown-out or dark areas. But when photographing brightly coloured subjects that are properly exposed, the iPhone’s HDR mode results in a disappointing desaturation. If the allure of your image is that it shows vivid colours then turn off the HDR setting.