So what exactly comprises an HDR image? Basically, more information per pixel. When you take a photo with your digital camera, the colors are converted to accommodate the limited palette of your display or a piece of photo paper. The human eye, however, is capable of taking in far more color and light information at any given time. This is why it's necessary to take a photo with the correct exposure settings—what your eye sees as a uniform scene with a balanced brightness and color range needs to be regulated to fit within the more limited range of your camera's sensor, or else the image will appear under- or overexposed (too dark or too light).
HDR provides a way to combine a range of exposures of the same scene into one image, adding significantly to the amount of data held per pixel (most digital images hold 8 bits of color information per pixel; an HDR image has 32). The result is an image with more "dynamic range"—in other words, the brights are brighter, the darks darker, and there's much more variance in between.
For a step-by-step guide to creating your own HDR images, continue reading below:
To get started, you'll need to shoot the same scene with a range of different exposures [above, my subject is the Williamsburg Bridge in New York City]. Scenes with uneven lighting really bring out the best of HDR (in my case, the bright lights of the bridges in the distance and the dark shadows of the cargo crane at left and the sky above). You can use the bracketing function of your digital camera (better point-and-shoots and almost all digital SLRs have it) to fire off three frames every time you squeeze the button—one with the correct exposure, one overexposed, and one underexposed. You want the difference to be as dramatic as possible, so if the three images look too similar, you can use the manual-exposure setting of your camera to take a series of exposures with a tripod (like I did here). The more exposures the better. And if your camera can shoot RAW images (an unprocessed format like a “digital negative” with greater flexibility), use that, as your images will have more detail.
Now, the magic. To combine them, you'll need software capable of doing the job. If you're using Photoshop CS2, you're in luck—HDR capabilities are built in. If not, there are alternatives. A cross-platform application called Photomatix Pro is a specialized HDR processor that costs $100 ($83 if you use the coupon code found here; there's a free bare-bones version just for Windows that I haven’t tested) and does an amazing job; since it only does one thing, it does it very well, offering specialized controls and batch-processing options that Photoshop lacks. There is also a rapidly improving, free, open-source alternative called qtpfsgui (great name, right?)—it doesn't have as many options yet for tweaking your HDR output, but you can't beat the price, and it's great to get started with. Venerable open-source Photoshop-alternative the GIMP has yet to incorporate HDR support.
Using whatever software you settle on, you'll need to combine your batch of variable exposures into a master HDR image. If you used a wobbly tripod (or worse, handheld your shots), you can have the software attempt to align them automatically—if they're not too far off, this usually works fairly well. The resulting image might take a while to generate and look a little weird when it does; this is because your screen isn't capable of displaying HDR images. To get the eye-popping HDR color effect, you'll need to downsample the image back to 8 or 16 bits per pixel, but in a way that blends the high dynamic range of your HDR composite image into one that still retains the increased detail and color range of HDR but fits comfortably in the viewable range of your monitor or paper. This process is called tone mapping.
Tone mapping is where the serious bit-crunching comes in, and each of the software tools detailed here has a different way of doing it. Photomatix provides a fairly straightforward dialogue of sliders that regulate the brightness, white and black points, and numerous other aspects of the resulting image—you can get some wild effects just by tweaking them and seeing what happens in the live preview. Photoshop gives you four tone-mapping choices. But the hands-down best is "Local Adaptation," which gives you control of the image via the "curves" control. I recently learned how to use curves, which are the basis of almost all digital-image processing, and I'm still not good enough to really explain them. I learned from here, though, and if you use Photoshop, your life will be better for learning as well. Anyway, this gives you great control of the image's color and exposure, and again, simply playing around and observing the live preview can yield some fun results. Qtpfsgui has all kinds of crazy-sounding tone-mapping functions to choose from (Drago logarithmic mapping! Durand fast bilateral filtering!); since no one but the mathematicians who invented them have any idea what they mean, trial-and-error is again your friend.
After you've found some settings that work (it's amazing the range of output you can get), voila, you've got your first HDR image. As you can see below, the difference between the correctly exposed normal image and the tone-mapped HDR output is marked: richer darks in the water, less blown-out whites in the lights, and more vibrant colors.
So now what? As you might expect, there are countless groups on Flickr dedicated to HDR where you can show off your work, seek feedback, and learn more in the discussion forums. My favorite is the largest (simply called HDR), but there are many others dedicated to users of specific software, people who go for a more realistic look with their HDR imagery, and so on. As you'll soon see, some people really love HDR and some people really hate it, but as with most things dealing with the visual arts, a lot of it comes down to personal aesthetic. No matter what your feelings on the HDR look, though, it's still pretty amazing to see how the process works, and more amazing still that anyone with a camera and a computer can try it out for themselves. Viva la digital revolution! —John Mahoney