Pro Techniques

Along the way, Ferrell made something of a breakthrough. "Most of the reading I did on HDR said that you can't use flash when you shoot HDR image sets, but I didn't listen. I worked out a technique for flash. I call it flash merging, and it involves using a flash off-camera, handheld, to spotlight various areas of a scene."

Ferrell used flash merging techniques for this image. "I took six images with the flash held in a different position for each exposure. The images were then merged and tone mapped."

Judging how many exposures to make of a particular scene is an important element of HDR photography. Ferrell calls that judgment "scene awareness," and his guidelines for judging scene contrast involve the direction of the light. "If the scene is frontlit, or the skies are overcast, the scene will have low contrast, and you can usually capture that in a short EV range--three images will probably do the trick." If the scene is sidelit, he classifies it as a medium contrast scene, and that would require capturing a greater EV range. Backlit scenes are, he says, pretty much the highest contrast scenes you're going to see in nature, and for those you'll need to capture a larger dynamic range. "You might go from three to nine images for a backlit scene or an interior, which can have a large range of contrast."

This is the result of six images taken with the flash unit held in a different position for each exposure. "I took care to fill in the shadows created from the previous flash exposures," Ferrell says, "so a fully illuminated scene would result after merging."

The imaging program that Ferrell prefers is Photomatix; he also has experience with Dynamic Photo HDR, FDRTools, and Artizen and has worked with Photoshop CS3's "merge to HDR" function. While it may sound like the imaging programs do all the decision-making, there's plenty of input from the photographer. In tone mapping, for example, you're given anywhere from five to 15 sliders to adjust, depending on the program. "Those sliders control the compression of the image," Ferrell says. "They bring out shadow detail and smooth the highlights, and there's a range of things you can control that are similar to the adjustments you'd have in Photoshop. The tone mapping decisions you make personalize the image."

What makes a scene an HDR candidate? "There are a couple of things," Ferrell says. "The contrast of the scene immediately indicates to me that the digital sensor can't capture all this--I need multiple images to get the full range of light. So it's typically an image that has sky and dark shadows. And HDR is great for grungy scenes--a junkyard, urban decay, graffiti, peeling paint; it just brings out those details."

For this image, a flash was fired from five locations around the glasses.

If you're interested in HDR, the best way to learn it is simply to do it: sit down at your computer with a set of exposures, the software program you've chosen, and an HDR guide. Ferrell's is one, but there are others, including Michael Freeman's Mastering HDR Photography; HDR: An Introduction to High Dynamic Range Photography by Jack Howard; and a DVD from Ben Willmore titled High Dynamic Range Mastery.