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Tuesday, July 30, 2013

Keeping It Real With HDR

How to use HDR software to maintain appropriate contrast, overcome the limitations of the sensor and bring out detail—without overdoing it!

This Article Features Photo Zoom

Clearing storm over the Sneffels Range at sunset, San Juan Mountains, Colorado.
2) Let highlights be highlights. As Margaret Livingstone, a neurobiologist and vision researcher at Harvard Medical School, points out, "We don't actually perceive the amount of light at any point in a scene, but instead we perceive the relative amount of light at each point, compared to that point's immediate surround." In other words, she adds, "Something looks light only if it's lighter than its background." Photographs of high-contrast scenes generally look more realistic if the brightest tones are close to pure white, but not clipped. Images containing the sun are an exception. The disk of the sun itself will always be blank white. Note that our eyes are subject to veiling flare, an overall diffuse, washed-out appearance, just as much as our lenses. Veiling flare occurs when the sun is shining directly on the lens surface, even if the sun is outside the frame. Allowing some degree of flare to reduce the contrast in the region around the sun can actually make an image look more natural. Your HDR sequence may have a frame that's so dark, it shows very little flare at all, but that's not necessarily the best frame to match.

3) Let shadows be shadows. Compressing the tonal scale until the shadows and highlights have the same density screams "HDR!" Retaining small areas of pure black makes your images more believable as long as the subject matter allows it. Most high-contrast scenes do. Large areas of near-black shadows, however, can be unnatural, since our eyes can usually see good detail in broad shadow regions.

4) Maintain good local contrast in highlights and shadows. Our visual system doesn't analyze contrast globally, that is, we don't look at a scene overall and call the darkest tone black and the lightest tone white. Instead, we analyze contrast locally, within regions, most notably highlights and shadows.

North Maroon Peak from the summit of 14,156-foot South Maroon Peak at sunrise, Maroon Bells-Snowmass Wilderness, Colorado.
According to both Mark Fairchild at RIT and Margaret Livingstone at Harvard, there's no easy way to define in a scientific sense what level of local contrast looks most realistic. Fairchild adds, "Amazingly, we are pretty good at just looking at them—images—and making a judgment—and with fairly good agreement among observers." When in doubt, go back to your bracketed set of images to see what level of local contrast you captured in the frame that's properly exposed for the region of interest. Remember that surrounding the target region with high-contrast subject matter will make the target region look less contrasty. Surrounding the same target region with low-contrast subject matter will make the target region seem more contrasty. Strong overall contrast tends to make bright areas look brighter. For example, if you have the sun in the frame, letting a few shadows go dark will make the sun appear brighter, which will, in turn, enhance the appearance of naturalness.

5) Look at HDR images at all scales, from thumbnail size to print size. Excessive use of Clarity in Adobe Lightroom or Camera Raw or Structure in Nik HDR Efex Pro 2 can look fine at screen size, but be a problem once you zoom in to print size at 16x24 inches. The Detail Contrast slider in the Details Enhancer tone-mapping method in HDRsoft Photomatix is a gentler control that's less likely to damage your image. Digital graduated ND filters can be useful tools, but if the transition zone is too narrow, they can make thumbnails look strange even when the print-size image looks fine.

6) HDR software packages have different personalities, which can create different problems. In my experience, flat local contrast in the shadows can be a problem with the Details Enhancer tone-mapping method in Photomatix.


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