Mix “˜em Up, Part 4: HDR imaging in Photoshop CS4

This is the fourth in a series of posts on digital darkroom techniques describing digital darkroom techniques that “combine” groups of images towards various ends. Previous parts covered focus blending and stitching, today we’ll talk about HDR (high-dynamic range) imaging.

High dynamic range (or HDR) imaging refers to a set of techniques for capturing and representing scenes where the range of brightnesses in an image exceed the range that your sensor can capture-if you’ve photographed many sunsets you’ve almost certainly had the experience of shooting scenes where you face the choice of either blowing out highlights or letting some significant parts of the image go entirely black. HDR imaging lets you take multiple exposures of the scene, say one exposed for the highlights and another for the shadows, and create a “high dynamic range image” (HDI). Unfortunately, combining the images is only half the battle, since your computer monitor or print has if anything a smaller dynamic range than your camera sensor, once you have a high-dynamic range image you’d need to tone map the image, that is, interpret the HDI back into a single lower dynamic range image. It is this interpretive step that is usually a challenge.

Exposure: Highlights blown
Exposure Highlights blown

Exposure: Shadows blocked
Exposure Shadows blocked

Creating and interpreting HDR images in CS4 is easy, open each of your component images, and then select File > Automate > Merge to HDR …, select “Add Open Files”, and press OK. Make sure you leave the option to align the images checked. The “Merge to HDR” dialog box will open, probably with a very washed out version of your image being displayed. At the right of the dialog box, underneath the histogram, adjust the single slider there until there are no blown highlights in the displayed image, then press OK. You’ve created your first HDI.

At this point you can edit the HDI in limited ways, for example, levels and saturation adjustments are available but curves and white balance adjustments are grayed out. You’ll probably notice that you’ve lost some saturation, go ahead and fix that at this point.

To “Interpret” a 32-bit HDI image back to a 8 “normal” image, simply select Edit > Mode > 8 bits per channel …. A dialog box will open, and this is where much of the magic will occur. This dialog box will offer you four “tone mapping functions”, and you’ll want to both experiment and read more about those options as you begin to play with HDR imaging, I’m not going to be able to cover them in depth here. The first three functions are “global” tone mapping functions, which operate, like levels or curves, on every pixel merely based on the value of that pixel. The fourth, “Local Adaptation”, works out the color and brightness of each pixel not only based on that pixel and the other settings, but also on the brightness of the surrounding pixels, brightening darker areas more than lighter ones. (If you’ve seen interesting, surreal “HDR images”, they were created with a local tone mapping function.)

Interpreted "HDR" result
Interpreted HDR result

Consider HDR imaging just one of several techniques that can be used to deal with high-exposure range scenes, alongside waiting for softer light, judicious use of fill-flash, or ND grads, each of which has their own strengths and weaknesses. It’s also possible, and I do this a fair bit, to put multiple exposures of a scene onto separate layers and to simply paint layer masks to merge them. But with practice, CS4’s “Merge to HDR” tool can help you produce incredible results from scenes that would be otherwise impossible to capture normally.

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