Extending Exposure with HDR

Just as shooting multiple exposures to blend into a panorama can help to extend the frame, High Dynamic Range (HDR) imaging techniques allow you to extend the limits of camera exposure and create images that contain a range of contrast and brightness values that better reflect how you see the play of lights to darks, which would be impossible to capture in a single shot. The ability to easily create and combine multiple exposures of a scene into an HDR image has been one of the most groundbreaking aspects of digital photography, freeing photographers from the technical limitations of the image sensor in their cameras.

When to Shoot HDR

In many scenes, the dynamic range of contrast from the deep shadows to the bright highlights exceeds the capabilities of image sensors to capture all the tonal detail present in the scene in a single exposure. High-contrast outdoor scenes on bright, sunny days; sunsets ; twilight photography; and interior locations that combine darker areas with artificial illumination or views through windows to brightly lit exteriors are all situations in which you can use HDR techniques to capture a full range of tonal values.

Because HDR involves taking multiple shots of the same scene, each at a different exposure optimized to record a different level of brightness in the scene, it is not suited for all types of photography, especially where fast motion in the main subject is an integral part of the story, such as with sports photos. It is an excellent match, however, for landscapes, cityscapes, architecture, nature, travel, and still lifes, and can even be used to create an intriguing look for highly stylized portraits. For longer exposure times, a tripod is necessary, but for scenes where the shutter speeds are fast enough for handheld photography or where you can artfully brace the camera to reduce camera shake, your camera’s Auto Exposure Bracketing feature will let you create HDR images of many different subjects.

Photographing for HDR

To make a great HDR image, the first step is recognizing situations in which this technique will work well for the scene. As noted previously, any highcontrast scene with a wide range of brightness values is perfect for HDR. But even scenes with tamer contrast ratios can be enhanced in interesting ways when rendered in HDR.

Exposure considerations :

Recording a scene in HDR involves taking a series of shots so that all levels of the brightness range are recorded with a good exposure. The number of shots you create will vary depending on the range of contrast present in the scene. Three to seven are a typical number that work for many situations, but more may be necessary in extreme situations, especially when the light source, usually the sun, is in the frame. The number of shots you use will be determined by the lighting conditions and also by how much detail in the deepest shadows you want to reveal. To get the most benefit from HDR, shoot in RAW to capture as much tonal information as possible.

Here are some additional exposure considerations to keep in mind:

Aperture. In terms of camera exposure, the main thing to remember is that, as with panoramas, the aperture needs to be the same for all the shots in the sequence so that the depth of field is consistent. Differences in depth of field will cause alignment issues when the images are blended together.

Shutter speed. Because the aperture will not change, adjusting the shutter speed will create the range of different exposures. If you will be using an Auto Exposure Bracketing feature and holding the camera, keep an eye on what the shutter speeds are for the different shots. If the shutter speed gets too slow, it may not be feasible to hold the camera for the shot without some trace of camera shake being recorded. Vibration reduction lenses can help you get by when holding the camera, even at lower shutter speeds like 1/15th and 1/8th of a second. But you should test the camera’s ability to record a sharp handheld shot at those slower speeds before relying on it for a photograph that really matters. When in doubt, try to steady the camera as best you can, and for best results, use a tripod.

Exposure range. The classic approach to HDR exposure is to bracket the shots so that they are one stop apart in exposure (Figure 6.22), although some photographers use a 2-stop difference between images. In a shot where the “normal” exposure would be f/8 at 1/125, a 1-stop range would result in shutter speeds of 1/30 and 1/60 on the overexposed end, which will record more detail in the darkest shadows, and 1/250 and 1/500 on the underexposed side, which will record detail in the brightest highlights. This would produce a sequence of five images with an exposure difference of one stop each.

ISO. If the camera is not on a tripod, the ISO should be set to a number that produces exposures with shutter speeds than can be adequately handheld. Onsite testing will determine the best ISO for handheld shooting. If you do have the luxury of using a tripod, choose a lower ISO, such as 100 or 200, to minimize noise.