Image capture for HDR imaging is slightly different than traditional single-shot low dynamic range photography. The goal is not necessarily to capture a single frame that best represents the scene before the camera, but to capture a series of different exposures that are intended to best capture the various tonalities contained within the scene.
In simpler terms: HDR imaging begins with a bracketed series of low dynamic range images that run the gamut from underexposure to overexposure. Perhaps counterintuitively, the underexposed source images from the series are used for the extreme highlight details, and the overexposed source images provide the shadow detail.
It makes perfect sense when you think about it for a moment. In the underexposed images, all but the spotlights, sun disk, or other extremely bright image elements will be thrown to shadows, but the highlight areas will have a degree of detail. In the overexposed images, all but the darkest shadows will be blown out and white-clipped. But those very dark shadows and low tones will appear close to normally exposed—providing shadow detail that would be lost in a normally exposed tonal range. By combining these extremely exposed source images, along with like images from the series that are exposed for the middle tonal areas, the tonal information gleaned from all photos provides the HDR generation program with enough information throughout the image curve to create an HDR output image that shows the expanded detail throughout the tonal range. This output is also in a bit space that can be displayed and printed.
How does one create the variously exposed source images? Depending on the specific camera, there are several methods for creating a bracketed series. Regardless of the bracketing method, the only exposure variable that should be adjusted is shutter speed. Changing ISOs may introduce noise pattern issues during HDR generation. Changing the aperture will also affect the depth of field, which will also cause image quality problems during HDR generation and processing. Change exposure only by changing shutter speed.
Flash should not be used for basic LDR source image capture, because the strobe will attempt to expose all the images properly.
Additionally, if possible, disengage autofocus from the shutter button, either via your camera's custom functions, or by using manual focusing mode. Even a slight focus shift between exposures will have a negative impact on the photomerged series.
Depending on the specific camera model, AEB should be an option in almost all shooting modes—Full Program, Aperture Priority, Shutter Priority, and Full Manual. To ensure aligned images with identical depth of field between exposures, it is best to use either Aperture Priority or Full Manual modes for LDR source image capture. If neither of these is an option, use normal Program mode.
It also makes sense to use a low ISO to minimize digital noise in the source images because the subject matter is still.
The simplest and easiest method is Auto Exposure Bracketing (AEB), which is a feature found on virtually every DSLR along with many compact digital cameras. Check your camera's user manual for brand- and model-specific instructions on how to set exposure bracketing.
AEB will shoot, at minimum, three photographs at a set exposure value offset (+/− 2/3, +/− 3, +/− 1.7) in either 1/3 (or .3), ½ (or .5) or 1 (1) stop increments. For simplicity's sake, we will use only fractional values throughout this eBook, but the concepts and techniques and exposure offset are identical even if your camera using decimal notation. (Note that in weird camera math "1/3" equals ".3" and "2/3" equals ".7", not ".666....") Also note that certain cameras work in 1/3 EV increments, others in 1/2 stops, and some high-end cameras offer the option of choosing 1/3 or 1/2 increments. The 1/5 stop differential between 1/2 and 2/3 is negligible, so don't fret about the numbers given in the following descriptions if your camera does not match these exactly.
Check your owner's manual for model-specific instructions on how to activate AEB:
Activate AEB, and select a number of images in the series (3, 5, 7, etc., depending on camera model).
Select an exposure value offset, 1/3, 2/3, 1, 1 1/3, etc. The greater this number, the greater the exposure difference between each captured frame.
Set your camera to "Burst" mode to capture the AEB series with one touch of the shutter button (optional.)
Check exposures and histograms in playback mode to ensure that the AEB series has enough variation in exposure range between the underexposed and overexposed shots. The "middle" (0 Exposure Compensation) photo should look very much like a normal exposure. For most scenes, this "normal" exposure histogram should follow a bell curve, with the peak near center.
If there is not a significant variation in the exposure of the AEB images, increase the Exposure Value offset. Repeat above steps.
That's the basic AEB workflow. Nice and simple. But, there are a few ways to expand AEB to create more LDR source images with your DSLR (and some compact digital cameras). Next up, advanced AEB techniques and tips.
What if you want to take images that capture an exposure range that is wider than your camera's maximum AEB setting, but aren't quite ready to tackle manual bracketing? By combining your camera's exposure compensation setting with AEB, you can cover more ground with two sets of AEB, shot one after the other. For example, set your camera's Exposure Compensation setting to its maximum negative value, −2, for example, and set AEB to this same numeric value +/−2. The +2 AEB image with −2 Exposure Compensation will actually be normally exposed-that is, not have any exposure compensation applied, as the +2 and −2 will cancel each other out resulting in a normally exposed image.
Now repeat this process, but set the Exposure Compensation to +2, and keep the AEB again at +/−2. Now, the −2 AEB image will be normally exposed, by the same formula as above. Presuming this is a 3 image AEB series, you've now captured 6 source images with an exposure range of −4, −2, 0, 0, +2, and +4. Discard one of the zero valued (normally exposed) images, and you've now created an LDR source image series of five images covering nine exposure values!
Another way to increase the number of source images with AEB is to change the bracketing amount and reshoot an AEB series of the same scene. Set AEB to +/−2 and capture three bracketed images. Then set AEB to +/− 1−1/2 and capture three more images. Repeat at +/−1, and +/− 1/2. Discard three of the zero-valued (normally exposed) images and you've created an LDR source image series of nine images covering five exposure values!
It's also possible to combine the previous two techniques to make a giant set of LDR source images, bracketed across a very wide exposure range, with only a subtle exposure difference between adjacent images. You may or may not want to use, nor need to use, every LDR image in such a big series, but experimentation is a big part of the HDR experience.
Don't worry, it's not the end of the world. Almost every digital camera in the market, from entry level compacts to pro-level DSLRs have an Exposure Compensation/EV function. Some variation on +/− usually indicates this setting, if it is a button-based function. On other models, it is a menu-based option, so check your camera's user manual for model-specific instructions on setting Exposure Compensation. You should also note that on many models, Exposure Compensation is not a selectable option in full manual exposure mode, since it's not needed. You should be able to use either full Program or Aperture Value mode to adjust the exposure.
Set your camera on a tripod or tripod substitute, as described above.
Select either Full Program or Aperture Value Program.
Turn the on-board flash to "Off."
Activate the camera's Exposure Compensation function and select the maximum negative exposure offset. Capture your underexposed image.
Set Exposure Compensation to zero and capture your normally exposed image.
Set Exposure Compensation to the maximum positive value and capture your image.
You have now captured a three-image series that covers a significant Exposure range by using Manual Exposure Bracketing. To create a source image series with more images, repeat above steps, but capture an image at more evenly spaced exposures throughout the camera's Exposure Compensation range: −2, −1, 0, +1, and +2, for example.
When shooting with a DLSR and certain advanced compact digital cameras, manual exposure mode combined with manual bracketing offers total exposure control over a very large exposure value range.
Using this method, it is possible to make a series of source images where all but the hottest scene elements are slightly underexposed and all but the darkest shadows are slightly overexposed, overcoming the EV range limitations of the Auto Exposure Bracketing methods described earlier.
The example below will yield an exposure value range of 11 stops with 1/60 as the middle point/normal exposure.
Mount the camera on a tripod.
Set the camera to full manual mode
Select an aperture and ISO.
Compose and focus the scene through the viewfinder.
Turn autofocus off.
Adjust the shutter speed until the in-camera metering is showing normal zero-balanced exposure and note the shutter speed, for example, 1/60.
Increase the shutter speed (which will lead to underexposure showing on the meter and in the resulting image) by five full stops, 1/2000 in this instance, based on the 1/60 given above.
Select a bracketing interval in either 1/3, 1/2 or full stops. For simplicity's sake we will use full stops for this example.
Capture the scene at 1/2000 (−5 based on a 1/60 metering) and adjust the shutter speed to 1/1000 (−4) and recapture the scene. Repeat in full stops all the way through to five full stops over the metered exposure value: 1/500 (−3), 1/250 (−2), 1/125 (−1), 1/60 (−0), 1/30 (+1), 1/15 (+2), 1/8 (+3), 1/4 (+4), 1/2 (+5). This will yield an 11 image series across eleven exposure values. Of course, it's possible to create a series of source images with more photos by shooting in 1/2, 1/3, or 2/3 stop increments across the same exposure range. Conversely, a shorter series can be captured by shooting in 2 stop increments across this same exposure range.
Depending on scene-specific captured information in the bracketed series, every image in the series may not be necessary to obtain the best results once the images are ready to be photomerged. However, shooting more source files across a very broad exposure range allows for more experimentation within the HDR programs. It's better to start with too many photographs than not enough!
Now that you've captured a series of LDR source images, it's time to transfer them to the computer and get your files organized. Organization of files and folders for source images is very important in HDR imaging. So important that it merits its own chapter.