HDR, or High Dynamic Range, is a method that allows you to take a picture of a subject with a brightness range that your camera’s sensor normally cannot capture. This is done by shooting identical images that have been taken at several different exposures. These are then blended together with the use of a photo editing program.
Although two separate exposures can be used to produce an HDR image, three exposures are generally recommended to achieve quality results. The objective is to have one exposure that captures highlight detail, another that has shadow detail and a third that covers the midtones. The easy way to take these exposures is to use the auto-bracketing feature of the camera. This will capture what the camera considers as the regular exposure, another that is underexposed to a degree and still another that is overexposed.
Auto-bracketing can often do the work for you but there are instances when you have to take manual control of the settings. An instance is when the multi-pattern metering systems of some cameras may detect the shadows of backlit subjects and compensate by overexposing the shots. In this case, you can make use of the spot metering mode or check the histogram for more information.
Most current digital cameras allow you to adjust exposure bracket intervals and exposure compensation, but usually up to +/-2 EV (exposure value). This may sound like a lot but you may need differences of 3EV or more.
If you want a quick way to take an HDR image, auto-bracketing could be sufficient. However, you might benefit more by using manual exposure since you have control over the adjustments. Needless to say your camera should have the manual mode option.
Spot metering is often used to precisely measure exposure. To do this, take a spot reading from the darkest shadow portion of the scene, then another from the brightest portion. You can then use these readings to measure the average of the two for the midtone exposure value.
Today’s cameras are equipped with features that can help provide exposure readings and the simplest is the histogram. If you have a compact or DSLR camera with Live View, turn on the histogram display. If your DSLR has no Live View, use the Playback Mode after you have taken your shot.
First, choose your lens aperture and keep it the same with all your bracketed shots. Let us say you are using an aperture of f/8. Only change the shutter speed when bracketing. For the ‘shadows’ exposure, adjust the shutter speed setting so that the left end of the histogram meets the left edge of the scale. The speed here could be slow such as 1/30sec. For the second exposure, set the shutter speed so that the right end of the histogram just meets the right edge of the scale. This will take care of the highlights and you might use a much faster shutter speed such as 1/500s. To get the midtones in the third exposure, adjust the shutter speed midway between the previous two you used. You do not have to be exactly in the middle and in this example, a shutter speed of 1/125sec would adequately capture the midtones.
Since we have kept the aperture size the same and changed shutter speed settings, these speed variations increase the chances of camera shake. A tripod will take care of this issue and will be most helpful in other ways as well. Good HDR images have merged exposures that are perfectly aligned and although HDR merging tools do a decent job of automatic alignment, it is preferable to get it right in-camera. With a tripod, you can change your shutter speed settings while the camera remains completely immobile.
Aside from camera movement, another thing to worry about is subject movement. There is a possibility that something in your scene might move or be moving, however slightly, and this can cause portions of your HDR image to come out looking blurry. People walking in and out of the scene or trees and bushes swaying in the wind can be problematic but there is not much you can do except to wait for the right moment or shoot duplicates of the three exposures. You can then choose which ones would be the best matches. If you are adept in using a photo editing program such as Photoshop, you could edit the bracketed shots to make them blend better into one quality HDR image.
Tags: exposure tips, HDR bracketing, HDR images, HDR technique, high dynamic range