|The key to creating an HDR image that brings the photograph closer to the reality that our eyes perceived is to maintain appropriate contrast and saturation. In the images on these pages, Glenn Randall created 32-bit files with Photoshop CS6 or Photomatix 4.2. He tonemapped in Lightroom 4. Above: Mount Sneffels in late September from County Road 7, San Juan Mountains, Colorado.|
One of the most frequent questions I'm asked by students in my landscape photography workshops is, "How do I make my HDR photographs look more realistic?" To answer that question, we first need a clear understanding of the problem. Our eyes can see a range of light intensities, from brightest highlight to darkest shadow, of something like 10,000 to one. A print can only display a range of light intensities of about 50 to one. In that sense, it's impossible for a print to ever look completely "real." But it's possible to get close—close enough that a print evokes many of the same emotions in the viewer that the real scene evoked in the photographer.
Aspen grove on top of Stealey Mountain at sunset, near Owl Creek Pass, San Juan Mountains, Colorado.
My guide when preparing prints has always been what I saw rather than what my film or sensor captured. I was never satisfied with the limited dynamic range of my 4x5 film, for example, and carried a quiver of eight graduated neutral-density filters to try to capture shadow and highlight detail the way I saw it. When affordable film scanning became available, I began using Photoshop to further adjust shadow and highlight density. Digital capture was another step forward, but even the high-end Canon EOS 5D Mark III I'm using today still has less dynamic range than my eyes, which is why I use a variety of digital techniques, including HDR, to capture what I saw.
But how do you really know what you saw hours, days or weeks later? According to Mark Fairchild, Professor of Color & Imaging Sciences at Rochester Institute of Technology (RIT), people are notoriously bad at remembering colors. We can distinguish thousands of different colors if they're placed side by side, but can accurately remember less than 100. We can easily remember if the flowers were blue or red, for example, but quickly forget what shade of pink we saw in sunset clouds. To further muddy the waters, people tend to remember colors as more saturated than they actually were, according to Fairchild. In addition, we tend to substitute certain "memory colors" for common objects. For example, we remember yellow-green grass as more green than it actually was. Similarly, we tend to remember sky as pure blue when it actually wasn't. Your best guide to what you saw, therefore, is your original bracketed set of images.
Let's assume you've loaded a bracketed set of images into your favorite HDR software. As you begin to adjust the 32-bit file, compare the region you're working on to the frame from your bracketed set that's properly exposed for that region. If you're adjusting the portion of the image that contains flowers, for example, "properly exposed" may mean the frame in which the green foliage surrounding the flowers was rendered as a midtone. If you're adjusting a sunset sky, however, don't pick the frame in which the glowing clouds are midtone and everything else is black. As pretty as those clouds may be, they're underexposed. Glowing clouds are a highlight; they should be brighter than midtone. Midtone clouds will look unnaturally dark if placed in a landscape where the flowers beneath are also midtone.
Here are some key principles for making your HDR photos look realistic.
1) Keep saturation under control. Wildly oversaturated colors may catch your viewer's eye, but excessive color saturation looks unnatural and is a flimsy reed on which to hang the entire impact of your image.
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.
Beaver Lake and aspen, near Silver Jack Reservoir, San Juan Mountains, Colorado.
On the other hand, when HDR Efex Pro 2 errs, it tends to produce excessive local contrast. If you're struggling to achieve a natural-looking result with one HDR package, consider a different one. HDR Expose 2 and HDR Express 2 from Unified Color are worth exploring.
7) Beware halos where dark objects meet bright backgrounds. This is another dead giveaway of an overbaked HDR. HDR packages have become much better over the past few years at controlling halos, but they can still be an issue, particularly if you compress the tonal scale too far.
8) When blending shadow and highlight regions into a believable whole, gradients can be your friend. Our visual system is very sensitive to abrupt change, but quite insensitive to gradual change. For example, we're sensitive to the abrupt change of color and density at the boundary between highlight and shadow regions, but rather insensitive to gradual changes in density within each region. By employing a digital graduated ND tool, such as the one found in HDR Efex Pro 2, you can place both the shadows and highlights relatively close to midtone, then blend them in a way our visual system finds believable.
Imagine a simplified example, in which the top half of the frame is filled by sunlit mountains and the bottom half of the frame shows shadowed flowers. You place a digital graduated ND filter over the image, with the transition zone of the filter centered over the dividing line between shadow and light, which lies in the middle of the frame. Now let's trace the changes in density caused by the filter as you examine the image from top to bottom. In the highest quarter of the frame, the filter has made the image uniformly darker. In the next quarter of the frame, which is still sunlit, the image gradually gets lighter as the virtual filter begins its transition from dark to clear. The beginning of the third quarter marks the beginning of the shadow region. The filter's transition from dark to clear continues in this quarter, so the shadow is darkened most at the very top of the quarter and not darkened at all at the bottom as the transition zone ends. The bottom quarter of the image is unchanged. Both the bottom and top quarters can be rendered close to midtone, but the transition from highlight to shadow still looks natural because there's still a strong color and density change at the shadow boundary.
In the days before digital, a "film aesthetic" ruled. A photograph was deemed "realistic" if it looked the way film originally recorded the subject. For a high-contrast scene, that meant bright highlights and inky black shadows. The invention of digital photography gave photographers greater control over highlight and shadow density, and the film aesthetic began to erode. Today's HDR software gives you total control over the density of every part of your frame, from the deepest shadows to the brightest highlights. This unprecedented power has completely overturned the old film aesthetic, but a new aesthetic hasn't yet become widely accepted. I've worked diligently to prepare a realistic-looking HDR rendition of a high-contrast scene, one that closely resembled what I saw, then had experienced photographers tell me they thought the result looked quite unnatural. During this period of flux, your best guide to "realistic" will be your bracketed set of images, your knowledge of how our visual system processes high-contrast scenes and your own good judgment.
You can see more of Glenn Randall's work, sign up for his monthly newsletter, read his blog and learn about upcoming workshops at www.glennrandall.com.