The Splendor and Misery of HDR

Zion AU09 HDR 4

HDR stands for High Dynamic Range. It involves images that contain the information of a very wide dynamic range, from the very bright to very dark. If one is shooting into the sun, one finds a dynamic range greater than what the camera can capture. The brightness of the sun is much brighter than the shaded portions of the image. The camera cannot capture the shaded portions without the sun washing everything else out; nor can it capture the sun without making the shaded portions too dark. HDR is a technique that enables one, theoretically at least, to capture the entire dynamic range of an extreme situation: the brightest hi-lights along with the darkest shadows. It’s easy enough to do. Just place the camera on a tripod and take multiple exposures and different shutter speeds. Expose for the brightest parts of the image and the darkest parts and everything in between. Then in post you combine all the information from the images taken at different speeds into a single file that contains the entire scene, with nothing lost due to under or over exposure.

Since ordinary image files are not large enough to store so much information, a special larger size file is required. Ordinarily, image files are either 8 or 16 bits. An HDR file is 32 bits. It contains twice as much data as a 16 bit file and four times as much as an 8 bit file.

Sounds promising. An image that captures the entire scene, no matter how extreme in terms of hi-lights and shadows. Only there’s a catch. It turns out that computer monitors cannot display 32 bit images, nor can printers print them. So the HDR file, in order to be usable, has to be translated back to a 16 or 8 bit file. In effect, we have to narrow the dynamic range of the image once again in order to make it displayable. The process is called “tone-mapping,” which involves trying to preserve as much of the original HDR as is possible, so that you have an image that looks sort of like what the eye sees when confronted with extreme dynamic range. That is, we want an image that looks realistic, with no burned out hi-lights and with details in shadows. Unfortunately, in trying to make this happen, issues arise. The most popular tone-mapping software is a program called Photomatix. Images tone-mapped with this program often manage to preserve the extremes at either end of a high dynamic image that is the goal of HDR, but not without producing other problems, such as unpleasant artifacts. Consider the sky in the following image:

Notice the magenta colored artifacts along the edge of the clouds. Also note the almost posterized rendering of the trees and hillside. Below is another example:

This picture contains the extreme dynamic range of the sun and a foreground in shadows. So far so good. The foreground is exposed fairly pretty darn well, considering the camera is pointed right at the bright setting sun. But the sky has an unnatural artifact bending across it.

Another issue is odd color rendition that sometimes plagues tone-mapped HDRs. Consider the following image of Fernbridge:

Note also the dark patches around the railings of the bridge. This is an example of “ghosting,” another big problem with tone-mapping software. In the following image, we see posterization and fringing problems among the rocks on the right side of the image:

Now HDRs don’t always have these issues. Sometimes, for whatever reason, they come out with much fewer issues:

And sometimes the effect of tone-mapping, even if doesn’t yield a perfectly realistic image, is nonetheless striking, as in the following image:

So HDR techniques lead to rather uneven results: sometimes stunning or at least intriguing, sometimes just plain bad. This is a technology barely climbing out of its infancy, so expect growing pains.