HDRI is a hot topic these days. It stands for High Dynamic Range Imaging, referring to techniques that capture a broader range of bright and dark tones than normal. HDR may sound intimidating at first, but it attempts to address a simple problem. Try shooting around sunrise or sunset and you will either have to expose for the sky or the foreground, not both. If you expose for the sky, the foreground will be dark. And if you expose for the foreground, the sky will be brilliant white. That’s because neither camera sensors nor film are capable of capturing huge differences in contrast.
So, what’s unique about HDR? It’s all about digital. With digital cameras, multishot techniques can be combined more easily with software to extend the dynamic range. The basic idea is to shoot as many frames as necessary to cover the contrast range in the scene. The main prerequisite is that the frames must be identical in every way except shutter speed, which you change to control exposure. Once back at your computer, special software is used to merge the frames into an output image. This results in a single image that has a more balanced tone range.
So far, so good you might say. But there are pitfalls. The HDR software used to blend photos tends to be complex, involving numerous toggles, switches and processing options. These features are needed because the software can make no assumption about your vision or intentions for the image. The photographer has to toggle the various program options to achieve a desired outcome.
I own three different HDR programs, and have experimented with hundreds of high dynamic range images. While I hardly quality as an HDR expert, I am keen enough to have noticed three main irregularities in the use of HDR software during my long hours of post-processing.
1) HDR software does not always produce a usable outcome, regardless of settings. I’ve seen many instances where the resulting image is so hideous that no permutation is salvageable.
2) HDR software can yield images that lack believability. This is great if the photo represents a fantasy land, but it doesn’t work for nature photography. In most cases, HDR techniques should be transparent to the viewer, unless your goal is to highlight the exaggerated effects.
3) HDR software can increase noise and reduce clarity. Be sure to zoom the final image at 100% to check for quality problems. In my experience, noise is often a killer problem.
Am I suggesting to avoid HDR software? Certainly not. But I do suggest you use it judiciously. Put HDR on a leash! Be willing to accept that it will not work for all images. At times, it does wonders and sometimes it fails miserably. And when it does fail, I always revert to manual HDR.
Manual HDR is my favorite approach for dealing with high dynamic range images. The capture process is the same as above but here the job of blending tones is done manually. The approach is easy for those well versed in Photoshop. The bracketed images are opened as layers, a mask is applied to each layer, and the desired tones are selectively painted onto the base layer using a brush. The result is a tone balanced image that looks realistic and is free of synthetic noise.
The big online bookstores carry several good titles on the subject of HDR photography. To get started, you may want to consider reading this HDR Tutorial by Trey Ratcliff. In addition, this article at The Luminous Landscape discusses a digital blending technique using Photoshop. Photomatix is currently the most popular HDR software, but another good alternative is FDRTools.