Tuesday, March 8, 2011
HDR Software Roundup And Review
The technology has matured beyond a gee-whiz bit of digital trickery into a full-fledged tool that nature photographers can use to overcome the limitations of the image sensor
Interest in high-dynamic-range (HDR) imaging has exploded over the last few years. More than half a dozen HDR software packages are now available, and for good reason, as HDR imaging promises to solve one of the great challenges of photography: taking the broad range of tones we see in the real world and compressing them into the much narrower range of tones we can reproduce on a monitor or on paper.
Until recently, I was an HDR skeptic, however. All too often, HDR software produced unnatural results. Shadow contrast sometimes looked strangely flat. Colors shifted in odd ways, and dark objects set against bright backgrounds often developed pronounced halos—weird bright bands along the object’s edge.
Granted, HDR software must solve a tough problem: replicating the intricacies of human vision. Our visual system analyzes contrast locally rather than globally. In other words, we don’t look at the darkest part of the overall scene and call it black, then look at the brightest part and call it white. Instead, we analyze contrast within regions, most obviously shadows and highlights, but also more subtly. The challenge for HDR software is to maintain good local contrast within regions, while still creating believable transitions between regions. Simply taking the overall range of tones and compressing it in some linear fashion won’t cut it.
The most difficult subjects for any HDR package are those that include the sun. Often, HDR software creates unnatural concentric bands around the sun, with sharp transitions between bands rather than a smooth gradient. For my first test case, I chose an image of the sun setting over Pigeon Peak, shot from the summit of Mount Eolus. For my second test case, I picked a high-contrast image of a stormy sunrise over Longs Peak. Using a Sekonic L-608 spot meter, I measured a six-stop range between the dark, shadowed midground evergreens and the glowing clouds. For each test scene, I made three exposures, with exposure compensation values of -2, 0 and +2.
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