This is a feature on more advanced cameras that people may not be taking as good of an advantage as they should be. The principle behind bracketing is to expose multiple exposures to make sure you get a good one. Lets look at a little history and camera behavior to understand why this is a cool features.
In the old days you had a choice to shoot slide film, which was a positive exposure system, that is the film ended up being what you saw, or you could shoot negative film in color or black and white which left a reversed image on the film. People shooting color film had a lot of help from the photo lab and they could salvage photos from the poorist of exposed negatives. This term is known as latitutde.
Most color print film today has a latitude of 5 stops of over exposure to 2 stops of under exposure. So if your exposure is any where with in 7 stops, the lab should get something printable. Black and White film doesn’t have as much latitude, but most people can over develop their film to help save the shadows and then use longer exposure times in an enlarger to get detail out of the highlights. So exposure does not have to be spot on with either type of negative film.
Slide film on the other hand is a different beast. Being over or under exposed by as much as a 3rd of a stop visibly affects the results. Photographers learned long ago, the benefit of shooting the same photo multiple times. In fact one of the uses of a motor drive on a camera is to shoot what’s known as “In camera copies”. You squeeze off 2 frames of your subject and you have a slide for you and one to send to your publisher.
The second issue is that photographers depend way too much on their in-camera meter to determine the exposure and they are simply not perfect. Understanding how meters work is key to understand why they are frequently wrong.
The meters built into the camera are called refelctance meters. They measure reflected light from the subject. They do not measure the light falling on the subject, just what is reflected at the camera. For many circumstances this isn’t bad. The camera reads the light and using various techniques attempts to get the photo to “average” middle gray, or what we call 18% gray.
The problem is our scenes do not always average out to 18% and bright or dark areas in the frame can fool the camera meter to think the scene is brighter than it really is, or darker than it really is. We know about problems shooting a snowy field or a sandy beach causing under exposure. But it can be subtle things like shooting a wedding with a white dress and black tuxes. Too much groom and your photo will overexpose, too much bride and you under expose. With print film, this isn’t that big of a deal because of its latitude, but shooting slide film in these situations you have to be spot on.
Well in our digital world, you need to think like your shooting slide film. Overexposure is deadly. Now our modern meters are really smart and can detect when your shooting at the beach, but it still has problems when you have your bride and groom settings.
Slide shooters solved this problem using a technique called bracketing.
Bracketing is simply shooting the same subject at a variety of settings then picking out the one that is best. You don’t need to have a camera with a bracketing feature to work. Simply either use your camera’s manual mode or use an automatic mode where you can dial in exposure compensation.
The basic principle is to shoot one frame at the exposure your meter suggests. Then depending on how much precision you want you will underexpose some frames, then overexpose others giving you a variety of exposures for the same subject. Then afterwards, you pick the frame with the exposure that is right and use it.
Most mid-range to pro level SLR’s and dSLR’s have support an automatic bracketing system. This is a button typically labled “BKT” or “ABE” (automatic bracketing exposures). When you put your camera into bracket mode, you pick the number of frames you want to bracket by. Then you set the amount of exposure change you want for each frame. This can be in 1/3, 1/2, 2/3 or whole stops typically. Some cameras will also let you set the direction of the bracketing.
I typically do a 3 frame bracket, at 0, +1 an -1 EV. Since I shoot RAW, I can easily compensate for minor exposure variants. Then I shoot three shots of the subject at the same zoom and composition before I move on to the next shot. Others may prefer to do a 5 frame bracket at 1/3 stop change that starts at 0 and moves downward 1/3 stops. Just remember to set the bracketing back to 0 frames to turn it off when you’re done.
Now for RAW shooters, bracketing isn’t quite as important as it is for JPEG shooters. On the film side, if your shooting slides you most certainly will want to bracket your exposures.