Expanding Tonal Range in Your Digital Photos
A scene like the one below presents the classic photographer’s challenge: Choosing exposure settings on your digital camera that capture the darkest parts of the subject appropriately causes the brightest areas to be overexposed. And if you instead expose for the highlights — that is, set the exposure settings to capture the brightest regions properly — the darker areas are underexposed.
In the past, you had to choose between favoring the highlights or the shadows. But now photographers have a couple ways to work around the problem:
- In-camera image manipulation: Some cameras now have tools that brighten the shadows without altering the highlights, enabling you to stretch a photo’s tonal range — the range of shadows to highlights, also called dynamic range.
Some Nikon cameras, for example, offer a feature called Active D-Lighting, which tackles the problem in two stages: First, the original exposure is slightly underexposed, to ensure that highlights are properly rendered. Then, before the image is written to the memory card, it undergoes a software process that brightens only the darkest shadows. This tool was used to create the improved seal image.
Some Canon cameras offer a similar tool called Highlight Tone Priority, and some Sony models offer DRO (dynamic range optimizer). Check your camera manual to find out whether you have this sort of option at your disposal.
- HDR (high dynamic range) imaging: This term refers to a technique in which you photograph the same subject multiple times, exposing some images for the darkest areas, some for the midtones (areas of medium brightness), and some for the highlights. You then use special HDR software to combine the exposures, specifying which parts of the frame to pull from which exposure.
For a great example of HDR work, take a look at the following two images, both from photographer Dan Burkholder. In the first image, you see the scene captured at a single exposure. The waterfall is beautiful, but you can’t see much detail in the shadows.
The second image offers the HDR version, created by combining the shot from above with seven additional exposures. With the expanded tonal range possible through HDR, you now can see the moss-covered rocks that the water is spilling over.
Some cameras offer automated HDR, capturing and blending multiple exposures with one press of the shutter button. The automated features usually don’t capture more than a couple of frames, and you don’t have much control over the exposure shift between frames or how frames are blended into the HDR composite. Still, they often produce better results than you can achieve in a single exposure.
The image below shows the type of results you can expect. This scene illustrates a problem often faced by real estate agents taking photos of their clients’ homes: How to capture both the interior of the house and the exterior landscaping that’s visible through the windows. The first two shots show you what was captured in a single exposure.
When the exposure was set based on the exterior, the interior was underexposed. When it was exposed instead for the interior, the view out the doors became too bright. To produce the final image, automated HDR mode was enabled. Is it perfect? Well, it would be nice if the interior was slightly brighter and the exterior was a tad darker. But it’s a definite improvement over the other two exposures.
However you approach HDR, use a tripod to make sure that the framing doesn’t change between shots — otherwise, the HDR software, whether in-camera or on your computer, can’t successfully blend the frames. Also, maintain the same f-stop throughout all frames so that the depth of field doesn’t shift from one frame to the next. Avoid scenes that contain moving objects, including people, in the blended HDR frame, moving objects will appear at partial opacity along the path they took while the shutter was open.