When you’re working in difficult lighting conditions—say, a contrasty midday scene, a backlighting situation or a scene that’s very dark or very light—it can be difficult to determine the correct exposure. In this situation, you might think it’s best to use automatic exposure controls, like program mode or shutter priority or aperture priority. But as smart as cameras are these days, there are still situations where they will get the exposure wrong.
In the old days, when working with film, photographers used a technique called “bracketing” to have a better shot at capturing one image at the correct exposure. You would shoot at the exposure you thought was right, then make another slightly overexposed and then a third slightly underexposed. This is bracketing.
Digital SLRs allow you to automatically bracket exposures. To set auto-exposure bracketing, look for a button labeled AEB or BKT on the back of the camera, or search for it in the camera menu. Most cameras offer three-shot auto-exposure bracketing, though some models allow for five-shot and even seven-shot brackets. For the moment, we’ll assume you’re doing a three-shot bracket. The methodology is the same on the larger ones, so if you want to go bigger and your camera will let you, go for it.
To set up auto-exposure bracketing, click the BKT button on the back of your camera, or find AEB, or Automatic Exposure Bracketing, in your camera menu. You’ll see a scale that allows you to establish the width of the bracket in full stops or fractions thereof. That means you can set the bracket for, say, +1 and -1. When you take the next three pictures, then you’ll have the exposure you’ve set, as well as underexposed by one stop and overexposed by one stop. (To be clear, you have to click the shutter three times to bracket the exposures.) The camera will remain in AEB mode until it goes to sleep or until it’s turned off; when it turns back on, it will once again be in the normal exposure mode.
You may think along the lines of “I’m pretty accurate with my exposures, so I’ll just bracket at a small increment, like a one-third or one-half stop.” And that’s fine, particularly if you were shooting film. But if you’re capturing RAW image files in your DSLR, you know that you have a fair amount of exposure latitude anyway before highlights get blown out and shadows are blocked up. As long as you’re pretty close, you’ll be able to dial in the exposure precisely during RAW processing. So that means I change my approach to auto-exposure bracketing when shooting RAW. I choose a larger interval, at least one stop and as much as two, in order to cover a wider range of exposures. With a two-stop RAW latitude, I can effectively cover a range of six stops by setting AEB to +/-2. A two-stop underexposure of a RAW file can still be shifted in post another stop darker; that’s the bottom of the six-stop range. And a two-stop overexposure will retain detail easily into another full stop of overexposure; that’s the top end of the range. That means with those three exposures I can effectively dial in a detailed exposure anywhere within a six-stop spectrum. Pretty impressive. And if you’re making HDR files, or you simply want the option of layering images to pull detail out of shadows or highlights, having three bracketed RAW files can be very helpful.
You can control how the camera adjusts the exposure during bracketing based on the mode in which you’re shooting. With auto ISO set and a manual exposure dialed in, the camera will adjust the ISO to bracket the exposures. In shutter priority mode, you’ll dial in the shutter speed and the camera will adjust the aperture. In aperture priority, the camera retains your selected aperture and changes the shutter speed to affect the bracket.
It’s a simple tool that offers a lot of ways to control your exposure and ensure you get the shot, even if you’re faced with particularly challenging lighting.