Tech Features

HDR: Getting more from your digital photographs

Jan 29, 2008, 11:47 GMT

Washington - Our eyes are far superior to any camera. With our eyes we can see details in darkness when a camera would pick up only black.

We can see tones in an overcast sky when a camera would see primarily white.

Put a camera in mixed lighting, with dark areas and light, and the camera will expose either for the light or the dark, but not really well for both.

That's why when you take a picture of a window indoors in the daytime, the areas around the window will largely be without detail.

Your camera exposes for the bright light coming in through the window and cannot simultaneously pick up the detail in the darker areas inside the room.

In short, the camera's dynamic range is limited. And that's where a digital photographic processing technique called 'high dynamic range,' or HDR, comes in. With HDR processing, you can turn many ordinary images into something truly special.

--- HDR: Getting started

Anyone with a digital camera can delve into the fun world of HDR processing. The idea behind increasing the dynamic range of digital images to produce spectacular results pivots around the ability of some software packages to combine two or more images that are identical except for the exposure of the pictures.

So before you can produce an HDR image, you would use your camera to take two or more differently-exposed shots of the same scene. Most people consider three identical shots to be a reasonable minimum.

To get the differently-exposed shots, you can take a couple of approaches. You could set up the camera on a stable platform or tripod and then use your camera's metre adjustment to snap off three consecutive images of the same scene.

Or - an approach that's easier - you could utilise your camera's ability to take pictures in its native, unprocessed mode, typically called RAW format. With a RAW file, you can later generate the differently exposed shots yourself by moving the exposure slider in the photo processing software that came with your camera. Some HDR software can do this for you automatically if you feed it a RAW file.

--- HDR software

To get from photo to stunning result in HDR, you need software that will digitally combine the differently exposed images into one photograph that contains a much greater dynamic range than could ever be achieved by one photograph alone. That's where HDR software comes in.

Today, a software company called HDRsoft is leading the way in HDR processing with a tool called Photomatix (http://www.hdrsoft.com). Photomatix comes in two versions: either as a standalone application or as a plug-in that you can use in the popular image editing program Adobe PhotoShop. Either way, you can download and try a free trial version.

But HDRsoft isn't alone. The latest version of PhotoShop itself contains a Merge to HDR feature in its latest version. And there are a number of free HDR tools now on the market, including the German- made Picturenaut (http://www.hdrlabs.com/picturenaut), from HDR Labs. But Photomatix is one of the few programs to offer the ability to automatically create differently exposed images for you out of one RAW file.

--- Creating the HDR image

Once you have some HDR software installed, creating an HDR image is essentially a two-step process. First you use the HDR software tool to open two or more differently-exposed images. When you do so, the software will take some time to process the images, combining them in a way that incorporates all of the exposure information from each file. What you'll end up with is one image on the screen.

At that point, you'll take step two, which is to use the tool's tone mapping feature to fine tune the image. Tone mapping allows you to adjust the lighting and tonality of the combined image. You have the opportunity in the tone mapping step to create a photograph with details that are dramatic, subtle, or somewhere in-between. Play with the controls in the tone mapping dialog box to achieve the results that please you the most.

--- Learning more

HDR is fun. And once you see the dynamic results you can obtain through HDR processing, you'll probably make it a staple of your photographic routine.

But you'll no doubt have questions along the way, and to get answers, you can turn to a number of online resources. The photography forum PhotoCamel (http://photocamel.com) has an entire section devoted to HDR photography, complete with video tutorials. Photo sharing site Flickr (http://www.flickr.com) has several groups devoted to the art of HDR. And a number of tutorials on specific aspects of HDR processing are scattered around the Internet.