Shooting a Realistic HDR Image
The High Dynamic Range Image
HDR imagery can be considered a recent fad or a well-worn technique, depending on how you look at it. In the early days, film failed to produce visible detail in the highlight and shadow values of high-contrast scenes. Because of this, photographers resorted to difficult and tedious darkroom techniques to help return the detail to the print. Today’s digital sensors, while holding great promise, still fall short of the capability of our vision. Luckily for us, however, modern computer programs provide a more accurate and elegant solution to this the age-old high-contrast problem.
The acronym HDR stands for High Dynamic Range. It can refer to a computer program, a photograph that has been processed by an HDR program, or the technique of taking multiple photographs with the intention of blending them together in an HDR program.
Let’s begin with the phrase ”dynamic range.” Dynamic range refers to the amount of separation between the brightest and darkest parts of a scene. A scene low in dynamic range would have a limited range of brightness tones, as seen in Figure 1.01. Here you see tones that are almost all the same brightness; the entire scene is made up of midtones. A scene high in dynamic range would have a large range of brightness values, as seen in Figure 1.02. Here the image consists of midtones, shadows, and highlights. There is a huge difference between the highlight values and the shadow values. This huge difference between values is what makes this scene high in dynamic range. High contrast is a more commonly used phrase to describe scenes with high dynamic range.
The problem with film and digital sensors is that neither is capable of providing a realistic image in these high-contrast situations. If you expose correctly for the shadow area of the scene, the bright highlight areas become overexposed (featureless white), as seen in Figure 1.03. If you expose properly for the highlight area, the shadows become underexposed (featureless black), as seen in Figure 1.04. In some cases, where the contrast is really extreme, it is possible to lose detail in both the shadow and the highlight areas. Both of these photos appear unrealistic because as we encounter these situations in real life, we see detail in the very dark and very bright parts of these high-contrast scenes. We see something more like the image in Figure 1.05.
Figure 1.03: A good exposure for the shadows results in overexposed highlights
Figure 1.04: A good exposure for the highlights results in underexposed shadows.
The HDR technique, then, is to take multiple photographs at different exposures. Each of these photos will capture a different range of detail. Once the photos are captured, you can then import them into an HDR program such as PhotoMatix. This program blends all the exposures into one photograph that contains full shadow, midtone and highlight detail. This resulting image is often referred to as an HDR image or HDR photograph.
Figure 1.05: An image created by blending the two previous photos together in an HDR program
When HDR Is Needed
The primary reason for wanting to shoot multiple exposures and blend them together in an HDR program is to capture full detail in a scene that contains very bright areas and very dark areas. These high-contrast scenes can be found everywhere, from landscape and nature scenes to interior architecture and real estate situations.
While important, shooting for HDR is not necessary with every photograph. Our cameras are capable of capturing the full brightness range of plenty of scenes. So when do you need HDR? The simple answer would be whenever the scene’s brightness range exceeds the camera’s capability to capture it.
By this measure, however, we would never have any photographs with pure blacks or whites, which are necessary to provide a photograph with full range of brightness levels (tonal value). Some pure black or pure white without detail is fine in almost any photograph. The image in Figure 1.21a and its histogram (Figure 1.21) show a slight clipping in the shadows. This is noticeable in the area around the waterfalls. As you can see, this small amount of pure black is perfectly acceptable. In fact, without it, the image might feel somewhat flat (low in contrast).
Silhouettes are another instance where you’ll want some pure black in your images. The image in Figure 1.22a and its histogram (Figure 1.22) show what we would normally consider severe clipping. Because there is no need to see any detail in silhouettes, however, the clipped shadows are just fine.
Figures 1.21 and 1.21a: An image and its histogram showing acceptable clipping in the shadows
Figures 1.22 and 1.22a: A silhouette image and its histogram showing the amount of clipped shadows
The mood of the photograph is something else you need to take into account. Not all images need to be presented as bright and full of midtones. A low key image is one that is dominated by darker tones. Not necessarily pure black, but just dark tones. Figures 1.23 and 1.24 are examples of this type of imagery. On close examination of these shots you can see there are areas of pure black, but they don’t fill the frame; they are interspersed with areas that are dark but contain detail. Compare these with Figure 1.25. Notice how the large area of dark dominates the frame.
Figure 1.23: Low key image showing acceptable amounts of pure black
Figure 1.24: Low key image showing acceptable amounts of pure black
While some areas of pure black complement an image by giving it a full range of values, large areas of pure black or white can overwhelm an image. This is the time for HDR. When you have large areas of pure black, as seen in Figures 1.26 and 126a, shooting multiple exposures and blending them in HDR is required to reveal detail in the shadows.
Figures 1.26 and 1.26a: Scene showing good highlight detail but no shadow detail
Once you’ve determined a scene needs HDR to bring out detail in the shadows or highlights, or both, it’s time to make the exposures that can be blended together. Shooting for HDR is more than just setting your camera on Aperture Priority, Evaluative metering, and Auto-Bracketing and firing off a few shots. Care should be taken to analyze the scene and set your camera accordingly.
Metering the Scene
Shooting for HDR boils down to making a series of exposures that capture the full range of tones present in the scene. The simple way of doing this is to get one good exposure for the highlights and then open up one stop (add more light via the shutter speed) and make another exposure. Then open up again and make another exposure. Continue this until the shadow areas are captured on the histogram. Figure 2.01a is an example of the first shot, where the highlights would be properly exposed. This was ¼ of a second at F/16. Notice the shadows are crawling up the left side of the histogram, indicating they are quite underexposed.
Figure 2.01b, shot at ½ second at F/16, would be the second shot. In this histogram both the shadows and highlights are clipped. Figure 2.01c was made at 1 second at F/16. The shadows are still clipped. Figure 2.01d was made at 2 seconds at F/16. The shadows almost have enough exposure, but not quite. Figure 2.01e shows the final image made at 4 seconds at f/16. Here you can see a histogram that represents full shadow detail.
Here is a visual example of six shots created to blend together in HDR. Figure 2.02 shows the images. Note that the brightest image shows plenty of shadow detail, and the darkest images retains highlight detail. Figure 2.03 shows the final combined image.
Here are a few tips that may help you achieve better results in the field:
1. Find the important bright area when metering. It’s essential to realize that not every bright area in the scene needs detail. Typically light sources themselves can do without detail. It’s also unreasonable to expect to get detail from the bright sun. Likewise, reflections from light sources in glass, mirrors, or metal should be ignored. Of course there are always exceptions. A very ornate lamp shade or chandelier will benefit from proper exposure. The main idea is to keep larger, important bright areas from blowing out.
2. Don’t concern yourself with the blackest black. Most images benefit from a pure black somewhere in the scene. Like for the highlights, determine which areas are truly important. Trying to get detail in every black and every white will result in an image series that becomes difficult to process correctly.
3. HDR software has the ability to blend together images that are not perfectly aligned, but it does take the software longer to produce the final results. If the images are too far out of alignment, however, the software may not be able to achieve perfect registration. While you might get lucky with a hand-held series of exposures, it’s best to ensure perfect alignment by using a tripod. The use of the tripod will also allow the use of smaller apertures for more depth of field.
4. Use a cable release or remote. Along with using a tripod, remote releases will help keep your images sharp by reducing camera shake.
5. Use the Self Timer. Some cameras will not shoot all of the exposures at once when set to Auto-Bracket. This means you have to press the shutter release button or cable release for each shot. While not terribly time consuming it would be nice if the camera would simply fire them all of with one press of the shutter. Try setting your camera to Self Timer. In many cases pressing your release once will trigger the camera to shoot the whole series of brackets automatically.
6. Consider using Continuous High Speed Release mode on a Nikon or Continuous Shooting mode on a Canon. By default, pressing your shutter release button shoots a single frame. In Continuous Mode, your camera will continue to shoot until you release the button. This mode can be used to capture a series of exposures in rapid succession, eliminating subject movement in between shots.
7. Many scenes don’t require exactly three-, five-, or seven-stop brackets. They might need four or six. In these cases it’s easier to set your Auto-Bracketing to capture more images than are necessary and delete the unnecessary images back at the computer.
8. If you are unsure about your metering or histograms, hedge your bet by capturing more images. It’s better to come home with extra images that are too light and too dark than to wish you had those images while you’re processing your HDR.
9. Shoot in RAW. RAW files contain much more information than JPEG files. More image information allows more options when it comes to blending your images together. HDR programs will process JPEG files, so uploading your old images is not an issue. For the most latitude in processing your images, however, set your camera to shoot in RAW.
10. I use the Program PhotoMatix Pro to blend my images together. I find this to be the most realistic and easiest program out there. It can be purchased from hdrsoft.com. Putting in my name, TimCooper (all one word, capital T and capital C) into the coupon code will get you 15% off the purchase price of $99.00!
This is an excerpt from Tim Cooper’s book The Realistic HDR Image from Peachpit Press. The Realistic HDR Image is an eBook that can be purchased for $8.00 by clicking here.
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