When to use the iPhone 4's HDR feature

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With its iOS 4.1 update, Apple rolled out a fun new feature just for iPhone 4 users—an HDR (high dynamic range) setting for the iPhone 4’s rear-facing camera. Dynamic range is the spectrum of light to dark that an eye, or a camera sensor, can read. Apple’s HDR setting takes three images at different exposures (underexposed, overexposed, in the middle) and combines them for an image with increased dynamic range. The resulting iPhone photos are closer to what the human eye really sees, with more details in the shadows and highlights than a standard image would have.

Because the iPhone’s HDR setting frequently results in improved images, it’s tempting to leave it 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). If one shot doesn’t wow you, you still have option B. Unfortunately, each HDR photo takes about 5 seconds to save. And if you’re short on storage space, saving an additional, somewhat larger version of each image file can add up fast.

The truth is that while this new tool can improve many iPhone photos, that doesn’t mean it is the best choice for every image you capture. Solve your time and space issues by learning what situations call for the iPhone’s HDR setting and when you should skip it altogether.

When to use HDR

Landscapes: A classic use of HDR in regular photography is on landscape shots with a bright sky above the horizon line and darker foreground below. iPhone 4 users who want to bring out more details in both parts of a landscape shot should turn on the HDR mode. One area that the iPhone’s HDR mode falls short is sunsets. In its attempt to lighten up the over-exposed sun, it also loses some of the beautiful red and orange coloring that makes the scene so stunning in the first place

Outdoor portraits: The midday sun’s harsh light usually makes for unflattering portraits. It can cast strong shadows on a person’s face and create dark circles around the eyes. It also bounces off of skin and accentuates shiny spots. The iPhone’s HDR setting can minimize the effect of these two extremes and create a more evenly lit portrait.

However, if the subject is completely back lit, the limited powers of HDR mode might not  be enough to brighten their face. To shoot a usable back-lit portrait, first tap to focus on the darkest part of the person’s face. The background will become extrememly blown-out. After you take the shot, the final HDR photo will combine the properly exposed person with a slightly toned town background. (Many photographers use a fill flash in back lit portrait situations, but unfortunately the iPhone’s auto mode adjusts for the presence of the flash and skews too dark.)

The top two images show a regular before HDR (left) and after HDR (right) back lit portrait. The bottom images show a before and after of an HDR image taken after tapping to focus on the subject's face.

Editing with apps: If you plan on using 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 less-poppy coloring, you can increase the saturation in a full-featured editing app such as Photogene or Adobe Photoshop Express. Fans of apps that imitate film should take into consideration what type of filter they plan on using. Some apps create their toy camera effects by bumping up the saturation and contrast—two things that are often decreased in an HDR photo. Filters that imitate old cameras can go the opposite way, desaturating an image for a faded look.

When not to use HDR

An example of ghosting in an iPhone HDR image.

Capturing motion: In HDR mode, the iPhone camera takes three photos in quick succession. If you’re taking a photo of a fast moving subject or if you move the iPhone while shooting, the final HDR image will show ghosting—that’s when the multiple images aren’t aligned and objects appear in multiple places. If you experience this problem frequently with your HDR images, consider mounting the phone on a tripod.

When contrast is key: A successful photo can create a sense of drama by contrasting light and dark. For example, an image might play up the impact of a strong shadow cast on a light surface, or of a completely black silhouette against a bright background. HDR shots will decrease an image’s contrast, diminishing its impact.

The sky in the non-HDR iPhone image on the left is bluer than the HDR image on the right.

Capturing vivid colors : The HDR mode can bring colors back into blown-out or dark areas. But when photographing brightly colored 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 colors, turn off the HDR mode. For example, if you’re taking a picture of a horizon where the blue sky is the focus and you don’t mind a dark foreground, turn off HDR and tap to focus on the sky to keep the vivid blues from being sucked out of your image.

When you need a flash: If you’ve played around with HDR already, you’ve probably noticed that the iPhone can’t use the flash when taking HDR photos. If you turn the flash on manually, HDR automatically switches off. If you’re using other lighting sources to light a dark scene, be sure to keep the camera steady or mount it on a tripod.

[Heather Kelly is a senior associate editor for Macworld.]

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