12 Sep
2012

Posted in: General, Photoshop, Tech Tips
By Ian Plant    19 Comments

HDR Images: The Importance of Preserving Relative Luminosity Tones

High dynamic range imaging—”HDR”—has become very popular in the past few years. Some might even say HDR is a fad, one that has arguably already come and gone. Of course, when people think of faddish HDR, they usually think of those “grungy” images spit out by programs like Photomatix. But HDR is broader than just the “grunge,” and it embraces many different techniques aimed at exceeding the camera’s dynamic range, including hand blending of multiple exposures using Adobe Photoshop (which is my preferred method).

Simply put, the human eye can simultaneously observe a far greater range of luminosity tones than even the best modern digital camera. DSLRs, on the other hand, can simultaneously record only a limited range of luminosity tones compared to the human eye. HDR involves taking several different exposures of the same scene and merging them together to exceed the contrast range of a DSLR sensor. Done properly, HDR can realistically capture a range of luminosity tones that approximates what the human eye naturally sees.

The problem (in my opinion) with many HDR images is that, in an effort to merge exposures, the photographer pays little attention to preserving the relative luminosity tones in the scene. For example, with many HDR images, areas in shadow are rendered as bright as or even brighter than the sky. The result? A photo that seems, for lack of a better way of putting it, uncanny—it just looks weird.

Think of it this way: when blending multiple exposures to create an HDR photograph, you are essentially “squishing” all of those real world luminosity tones (from deep shadows to bright highlights) into the range of tones that your computer monitor can display. In a perfect world, your HDR process would squish the tones together much the same way you would squish a Slinky: all of the tones would stay in the same position relative to each other, they would merely get squished together tighter. Thus, under the “Slinky” model, your bright sky would still look brighter than your dark shadows. Unfortunately, many HDR programs, and many poor attempts at manual blending of exposures, behave less like a Slinky and more like they were made out of LEGOs: when squished together, the relative tones break apart and get jumbled, and it can be rather easy to reassemble them out of order. Under this “LEGOs” model, your shadows end up looking brighter than you sky, and your photo ends up looking weird.

When merging multiple exposures together to create HDR images, it is important to keep relative luminosity in mind—in other words, it is important to make sure your process resembles squishing a Slinky more than reassembling broken LEGOs. Pay close attention to the relative tones in your scene. For the image below, I merged several exposures to capture the full dynamic range of the scene. I was careful to keep the brightest parts of the sky brighter than the reflections in the water, and the reflections in the water brighter than the foreground which was mostly in shadow. I resisted the temptation to unduly brighten the dark areas of the photo, including the trees in the background. Instead, I opted for a “moodier” look, as I felt it enhanced the stormy quality of the scene. Overall, my goal was to preserve as best as possible the relative luminosity tones in the scene, and present an HDR image which looked realistic.

So remember the Slinky when blending multiple exposures to create HDR photos. By preserving relative luminosity tones, your resulting image will avoid that uncanny “grungy” look, and instead will appear natural and unforced.

You can learn more about my exposure blending techniques, as well as many other Adobe Photoshop techniques which are a significant part of my personal digital editing work flow, from my Creative Digital Processing series of video tutorials.

About the image: This image shows Lake Superior in a very bad mood. Large waves crashed all around where I was standing, but I managed to find a spot which was relatively protected. Still, each incoming wave washed water up to my knees, so I kept an eye out for any “rogue waves” which might threaten me or my equipment. Every now and then, a wave would hit the curved rocky shore behind me just right, and send the water over the rocks in my foreground, pushing water into the scene and creating leading lines that encourage the eye to travel from foreground to background. This created a strong visual element in the lower left side of the image, which I desperately needed to balance against the bright eye-catching sunbeams in the upper right. I named the image after the native Ojibwe name for Superior, which means “Big Water.” Canon 5DIII, 14mm, polarizer filter, ISO 100, f/11, 1/4 second.

About Ian Plant (334 Posts)

Ian Plant's photographs and instructional articles have appeared in a number of books, calendars, and magazines, including Outdoor Photographer and Popular Photography. Ian writes a regular blog column for Outdoor Photographer online, and he is the author of numerous instructional eBooks and digital processing tutorials. Ian leads several photo tours each year.