Capturing a High Dynamic Range (HDR) Image
The need to shoot HDR is based on a limitation of the camera sensor to capture all of the information in a scene in a single exposure. The amount of information from shadow to highlight that can be captured in a single exposure is called the “dynamic range” of the camera. The problem is, in many situations the dynamic range of the scene that you are shooting has more information (a wider dynamic range) than the camera can capture in that single exposure. An example of this is when you capture a photo with good exposure in one area, but details are lost in another area. These lost details will either be in totally black or totally white areas of the image, and are referred to as clipped details (see Figure 1-1). The solution to this issue is to shoot multiple exposures at different shutter speeds to capture the full range of information in the scene.
Often times, photographers wonder how many exposures are needed to capture all of the information in a scene. There is no single correct answer; it may be any number of exposures based upon the scene that you are photographing (in the previous scene we needed 4 exposures 1 stop or EV apart from each other). There is a simple method to check to see if you have captured all of the information. When shooting your exposure series, pay attention to the histogram on the back of the camera. If you are clipping information in either the highlights or the shadows, as displayed in the histogram, this means you have not captured the entire dynamic range of the scene. To fix this, shoot enough exposures, increasing your exposure, until you have an overexposed image that is not clipping the shadow information. To make sure to capture all of the highlight detail, keep making under exposed frames until the histogram shows that the highlights are not clipped. Once you have at least one exposure that has no clipped details on the right hand side of the histogram, and at least one exposure that has no clipping on the left hand side of the histogram, you have created an exposure series that has captured the entire dynamic range of the scene. (figure 1-3 and figure 1-4)
Much like the number of exposures that are needed to capture a HDR photo, there is no perfect number of stops to change between each capture in the exposure series. The amount of stops will depend on the type of camera, the lens, and the scene being photographed. The typical guideline would be to change each exposure either by a single stop or by two stops. A single stop exposure will generally yield better gradients and transitions between exposures, reducing the chance of haloing. A two stop change can capture the same dynamic range with fewer frames, but will not yield transitions the same way.
There are a few additional tips to shoot a great exposure series. When shooting the photographs, it is important that the composition remain exactly the same so that the images align properly when merging. Best results come from shooting with a tripod and a cable release, or wireless remote as these tools will greatly reduce the chance of moving the camera between exposures. It is still possible to shoot a HDR image without the above mentioned equipment, just do your best to keep the camera as steady as possible.
There are various settings in a camera that can assist with capturing a HDR image. The auto exposure bracket mode and the continuous burst mode, if bracketing doesn’t enable it automatically, are helpful to be able to shoot the entire exposure series quickly. Capturing the shots as quickly as possible will help to reduce movement and composition changes when shooting handheld exposure series. When shooting, either use an aperture priority mode, or full manual mode to keep the aperture the same but change the shutter speed. Autofocus can be used but it is suggested to use only when setting up the shot. Turnoff the autofocus before you begin to shoot so that the camera does not try to refocus between exposures in a series. Lastly, set the white balance as close as possible to neutral or desired color balance when shooting the exposure series, then tweak as needed within RAW processing software. Shooting in RAW format enables you to adjust the white balance as needed after capture.
By applying these techniques, you will be able to capture the full dynamic range of the scene so that you can best capture the photograph you have in mind. Then, when you merge the photographs together, you have all of the necessary information to create the look and style of the HDR image you want.