There are times when it can be difficult to decide on what the ideal exposure would be to get the best image of a scene, especially in high contrast situations where the spread between the lightest and darkest is over 6 stops. It may be that you don’t have the time to think about your exposure. Or, it may be that there are elements of extreme brightness and shadow within the picture that you want to capture, and you’re not sure whether exposing for the highlights or the shadows will give you the better final image. Well BRACKETING could be the solution.
Bracketing is the technical term for a sequence of frames of the same image, shot in rapid succession and all at different exposures. Normally, it is a sequence of 3 or 5 frames with each exposure differing from the other frames in steps of between 1/3rd of a stop up to a full stop. Each sequence consists of a central exposure the camera deems to be the ideal exposure for the overall scene, one image under exposed and one image over exposed. Hence the ‘correct’ exposure is bracketed (or sandwiched) between 2 exposures which are under or over exposed by the same amount.
While DSLR camera users have the option to manually bracket between exposure settings, compact cameras (and DSLR users) have a built in control known as Automatic Exposure Bracketing (that is quite a mouthful so you may see it shown as AEB inside your camera menu). AEB lets you select how much variation you want between frames and then fires off 3 frames in quick succession once the shutter is depressed.
The first image of the sequence is centered round the exposure the camera has determined will be the optimal exposure to produce the best image, so this is the first picture frame taken. It then takes the same picture but with less exposure, and finally the last frame is given more exposure than the first. With a DSLR, you can set what that shift in exposure will be. This will give a series of 3 images, all of the same subject but with different amounts of shadow and highlight detail in them. The exposure variation that can be set between picture frames can vary between a third, two thirds or a full stop of exposure.
So when we expose any image, we are trading off losses in some of the shadows and highlights to gain the most acceptable exposure overall, regardless of whether we make the exposure decision ourselves or allow the camera to do it for us.
So what is the point of bracketing? Bracketing gives photographers leeway to take and combine these multiple images in photo-editing programs (called HDR – High Dynamic Resolution) to produce the ultimate perfectly exposed final image. Photographers are able to replace areas of shadow and highlight detail that could not be recorded with the main tonal range of the subject because the extremes of exposure went beyond the sensor’s dynamic range.
Bracketing can also give you subtly differences of exposures and allow you to choose what exposure compromise you are happiest with. Some photographers prefer to lose a little detail out of the shadows to keep the highlights from blowing out and becoming featureless white areas. Others prefer to see detail more in the darker tones.
If you are going to combine images into an HDR image, be sure to shoot off of a tripod and keep the depth of field the same between images in a specific sequence. So the next time you’re at a loss about your exposure, try a little bracketing. You never know, you might like it!