Improving the dynamic range that my digital cameras can capture is, for me, more important than getting higher resolution or more megapixels. A well-known technique for capturing more dynamic range is taking multiple captures of the same image using different exposures (i.e., bracketing). From these multiple exposures you then can create HDR (high dynamic range) images. I won’t explain HDR itself in more detail because it was covered recently (see “High Dynamic Range Photography,” by Dan Burkholder, PT, September/October 2007).
Unfortunately, the enemy of this technique is any sort of movement—whether of subject matter or camera. That is why exposure bracketing is usually only applied to static objects, and why the camera must be mounted on a sturdy tripod. Both conditions impose severe limitations. If these multiple exposures could be captured instantly, however, both issues would pose no problem. In the future, we actually may see cameras that can capture multiple exposures and still freeze fast- moving objects; for now, we may be getting close enough.
Recently, I received a Canon EOS-1D Mark III camera for review. This camera can capture up to 10 fps, and its target audience is sports and wildlife photographers. (I usually photograph more static nature or urban landscapes.) Once I got used to the 1D Mark III, I started experimenting with what I call high-speed HDR.* I set the camera to its highest burst rate and captured bracketed shots. That means a three-shot (-2, 0, +2EV) bracket sequence was shot in less than half a second. Why does it make a difference? Everything that does not move much in a half-second is frozen in all three exposures. It limits the camera movement and also captures all slow-moving objects reasonably sharply. I used this technique for at least 300 shots, and it started to change my photographic style.
Beyond “bad” light
Photographers talk a lot about “good” and “bad” light. We’re not really commenting on the light itself; we mean that some sorts of light are not good for our photos. It is bad light because normal film or digital cameras cannot capture the dynamic range in the scene. Using HDR techniques (multiple exposures and tone mapping) you can capture quite a bit more dynamic range. I’m now using high-speed HDR in many situations that I previously called bad light and have been getting very pleasing results.
I exclusively use image-stabilized zooms (most of the time in the 70–200mm range) and shoot handheld or with the aid of a monopod. I set the ISO range to a value that allows me to keep even the longest exposure in the bracketed shots sharp if shot handheld (taking the image stabilization into account). This is actually like a classic bracketing technique. It allows what I call “mix and match.” One can use the best single exposure or combine two or three images into an HDR file.
After some experiments, I started with a sequence of -2, 0, +2EV (Figure 1). In tools such as LightZone, you may even be able to rescue the +2EV overexposed photo for use alone. I prefer to combine all three shots in Photomatix and create an HDR image. The downside of using this technique is that I need more space on my CF cards and disks, as well as more time for image processing.
You can create HDR images in recent versions of Photo- shop. In fact, Photoshop CS3 introduced an excellent alignment algorithm, and I sometimes use CS3 for this too. Unfortunately, I have found situations in which CS3’s Merge to HDR function produces nasty artifacts, so I use Photomatix. Also, tonemapping in CS3 is quite tedious and not as much to my taste as that in the Photomatix Detail Enhancer.
Once you have an HDR image, you need to use “tonemapping” to fit the high dynamic range into a contrast range that can be printed. In fact, tonemapping is at least as important as capturing the full dynamic range, because without tonemapping, HDR images would not be helpful for making prints. Tonemapping also can be used for single, low-dynamic-range images to open shadows, tame highlights, and extract details.
I actually use a two-step tonemapping process. I first do a tonemapping in Photomatix, using the After Detail Enhancer. I am very careful during tonemapping not to clip any high- light data, and to try to preserve as much shadow detail as possible, staying in 16-bit mode the entire time. Then I fine-tune the photo in LightZone, mainly using the powerful Relight tool, which is a top-class tonemapper and detail extractor. Following this process, I created the image in Fig- ure 2 from the bracketed images in Figure 1. I also performed a perspective correction in Photoshop using the Crop tool. I had enabled the Canon 1D Mark III’s Highlight Priority mode, which offers about one f-stop more latitude in the highlights. There is likely slightly more noise in the shadows because of this, but this shadow noise is easily removed using multiple exposures.
Challenges of high-speed HDR
A second session that I’ll share used a different bracketing, and illustrates some of the challenges of using high-speed HDR. This time I only took two shots: -1 and +1 EV. In many cases one might be able to recover the scene from just the +1 EV exposure shot (thanks to the Mark III’s Highlight Priority mode). If not, the -1 EV shot is a good backup. However, combining both shots is likely the optimal solution if no fast- moving objects are in the shot.
There are a couple of issues to watch for. Some are common problems—not only an HDR challenge—but the tonemapping algorithms tend to amplify some of these artifacts, particularly chromatic aberration.
Most lenses show some chromatic aberrations (mainly zooms or cheap lenses). After tonemapping, which often makes it more noticeable, chromatic aberration can be quite disturbing. I watch for magenta or green outlines in my exposures, and correct them in Adobe Camera Raw, giving me less chromatic aberration using Photomatix HDR and the tonemapping process.
As a positive side-effect of HDR generation, shadow noise in the final image is reduced by about two f-stops, due to the shadow details being taken from the overexposed photo. Because tonemapping and, even more so, LightZone’s Relight tool brighten the shadows considerably, this reduction in noise is not just welcome, it is essential.
Let me describe my full workflow in creating the HDR image of the warehouse. First, I converted the Raw files to 16-bit TIFFs in Adobe Camera Raw 4.1. I performed the chromatic-aberration removal, but otherwise kept the images very soft in contrast and applied no sharpening.
Next, I generated an HDR file from these TIFF files in Photomatix. The source images have to be aligned, but you can do this automatically in Photomatix (and in Photoshop CS3). Because of the high frame rate, the shots are often only a few pixels off using a very fast camera such as Canon’s EOS-1D Mark III. I next corrected lens distortion (I use PTLens on Windows or LensFix in Mac OSX) and the perspective (in Photoshop). Finally, I used Relight and did further tuning in LightZone, and desaturated the blue sky.
I think high-speed HDR is a new and powerful technique. Being able to capture good photos freehand and in normal sunlight opens new possibilities. Besides requiring more space for images, I don’t see a lot of disadvantages. If some of the objects move between exposures, I still can use the best single exposure. I intend to continue to experiment with different bracketing sequences:
• -1, +1 EV: saves space
• -1, 0, +1 EV: would allow getting a good optimal single exposure that cannot be more than 1⁄2 f-stop off.
• -2, 0, +2 EV: if you want to capture an even larger dynamic range. The +2 EV exposure may often need the use of the tripod, however.