Unbelievable Madagascar September 14–October 1, 2007
				India Wildlife: On the Trail of the Tiger November 7–24, 2007
				Angkor Wat & Bagan—Temples of Cambodia and Myanmar January 17–31, 2008

All photography, including digital photography, involves compromises. No film or file represents color perfectly. Without manipulation, there is no such thing as infinite depth of field. And all photographic technologies compress dynamic range, the range of light intensity from pitch black to supernova. Color slide film can “see” about five stops, which means that it clips the brightest or darkest part of an image, or both, by turning the extremes into pure white or black, respectively. Most digital cameras capture about the same range as slide film or a bit more. Print film can record an additional stop or two, and black and white film can do one stop better than print film. However, the human eye can see 11 stops.

With the advent of Photoshop and similar programs, it has been possible to combine two or more exposures to increase dynamic range. This demanded masking or a savvy use of blend modes, but Photoshop CS2 introduced a tool to automate exposure blending called Merge to HDR (High Dynamic Range). With HDR one can create an image with more than 11 stops.

HDR is Photoshop’s first 32 bit tool. The 32-bit files are twice the size of 16-bit and four times 8-bit. They are capable of wider dynamic range. Unfortunately, monitors and printers can’t display all the range and colors 32 bit offers, yet. When they do, digital photography will take a giant leap forward.

Until that day arrives, we can confect a high dynamic range 32-bit image in the computer and convert it to 16 or 8 bit, expanding the apparent dynamic range by bringing out shadow detail and cooling off highlights. Here’s how:

First, take a series of pictures exposed one to two stops apart in a range that captures both shadow and highlight detail. Since the images should line up exactly, use a tripod. HDR has an alignment tool that may work if you must handhold, but a tripod delivers images with perfect registration. Use a cable release so camera shake doesn’t degrade alignment.

Make sure nothing is moving. A breath of wind can move leaves or grasses enough to foil the program. Rivers, traffic or any other movement defeats HDR.

Try at least three exposures. Don’t change the focus or the aperture from shot to shot. Any change of the center of focus will mess up the alignment and confuse the program. So, shoot with Aperture Priority or manually by changing only shutter speed.

Once you have your images, download them to the computer. You can Merge to HDR with RAW files in Bridge or in the Automate function of CS2’s File menu. When shooting RAW files, turn off the automatic exposure and contrast adjustment settings in Bridge so the HDR can work with the images as the camera captured them.

If working with RAW files in Bridge, go to the Tools menu, highlight Photoshop, and choose Merge to HDR from the flyout menu after selecting the images to merge. If you start in Photoshop, go to Automate in the File menu and select Merge to HDR.

These are the steps I use when working with RAW images of two stationary animals in Bridge:

I auto-bracketed three exposures using Aperture Priority and then highlighted the RAW files in Bridge.

First, I took three pictures exposed a stop and a half apart. The best exposure suffers from excessive darkness, but detail still appears in the shadows. I didn’t need more exposures because the tonal range wasn’t too broad. I shot in Aperture priority to keep the depth of field constant.

Next, I imported the files to Bridge, highlighted them, selected Photoshop from the Tool menu at the top of Bridge, and clicked Merge to HDR from the flyout menu. I chose 32 bit from the menu and hit “OK.” The first HDR Menu appeared with the three shots on the left and an initial composite in the main window. The histogram on the right shows the dynamic range of the 32 bit composite. No monitor can display such wide dynamic range, so the histogram comes with a slider to view “slices” of the composite file. You can move the slider across the histogram to confirm that you captured the desired range of tones. The program saves 32 bit files as Portable Bit Map files (.pbm). It’s a good idea to save the .pbm file so you have an image with the maximum information if you decide to start over at some point.

The first HDR menu displays the exposures, the HDR composite, and a 32-bit histogram.

Now comes the art of HDR. I fiddled with the tools in HDR’s tonal adjustment menus to find what worked best.

Experiment with the HDR file. Deselect one of the original exposures and see what happens. If the composite image looks better, fine. If not, reselect the exposure. There are several tools in the Method menu within the HDR Conversion window. Play with the Exposure and Gamma sliders (Gamma acts like a contrast control). Highlight Compression squeezes the highlight and Equalize Histogram starts to compress the entire image extremes. Any of these tools can help any given image, but they can also produce awful results. Usually small adjustments are best, but go overboard at first to learn how they affect the HDR file.

Local Adaptation acts a little like Shadow/Highlight in Photoshop. I find this to be the best tool for perfecting the image’s contrast. First, go to the Curves dialog. Create an “S”-shaped curve as a starting point by dragging the diagonal line down near the lower left and up near the upper right of the line. When you release the line, an anchor point is established. From that point, move other parts of the line up and down to see if the contrast improves. Once satisfied, set Threshold and Radius. Start with a low Threshold around 0.5. Then, experiment with the Radius slider. I used a radius of 8. If the radius is too big, you will see distracting halos around edges. When the image looks as good as you can make it, click OK to apply your changes. Save the result in 16-bit to retain as much information as possible and continue adjustments on the compressed file in Photoshop if you wish.

In this example, I made small changes in the Gamma and Local Adaptation menus.

The final result, after some Gamma and Local Adaptation work.

Although HDR can perform wonders, it is a long way from automatic. Sometimes it generates strange results, usually when it works at the extremes of dynamic range. The 32-bit file is fine, but things can get wacky when the program has to discard information to compress the image to 16 or fewer bits. Just play with it to get a feel for how the tools work—with practice you will be able to capture images with greater tonal detail that ever before.

Be sure to check out James' latest book, DIGITAL PHOTOGRAPHY OUTDOORS: A Field Guide for Adventure & Travel Photographers, published by The Mountaineers Press.

All images Copyright © James Martin