What is Auto Bracketing?
Auto bracketing (or exposure bracketing to be more precise) means that you set your camera to take multiple images with different exposures. Generally you would set the camera to auto bracket 3 images at ±1ev. This means you get one image underexposed by 1 stop, your normally exposed image, and then one image overexposed by 1 stop.
It is also possible to bracket other camera settings, such as White Balance, but generally when someone refers to auto bracketing, they mean bracketing exposures.
How digital cameras work out auto exposure (a very brief overview)
Before I go into why I always use auto bracketing, here's a quick brief on how digital cameras decide to expose the image when you are using an Auto mode (Aperture Priority, Shutter Priority, or Program Mode).
Traditionally Cameras' meters would be set so that the average Luminance of a scene would be equal to Middle Gray. This meant that if the scene you were photographing was quite bright e.g. snow or the sun shining through clouds, the photograph would come out quite dull and underexposed. Of course, you'd also get the opposite problem when shooting a dark scene.
Nowadays digital camera meters are much more advanced and actually have a database of scenes with appropriate exposure values programmed into them. When metering a scene the camera looks at its database to try and find the correct exposure based on a scene similar to what it thinks you are photographing. The Wikipedia article on Metering Modes covers Multi-zone metering in more detail if you are interested.
Still, even with the sophisticated methods that modern digital cameras can use to determine the correct exposure for a scene, you'll often find that a scene is either too dark or too bright. This is why I auto bracket all my images.
How to set Auto bracketing on your digital camera
Setting Auto Bracketing depends on your camera model, in my Canon 450D DSLR you have to go into the Shooting menu to set it, while my Nikon D200 DSLR sets Auto Bracketing by pressing a dedicated button and using the thumbwheel to change the Auto Bracketing settings.
The benefits of exposure bracketing are:
I find that sometimes images look different on my monitor to how they display on my digital camera's LCD. Sometimes they will look brighter on the camera's LCD, sometimes darker, it depends on the lighting conditions. In bright sunlight I can't actually see the images on the camera's LCD at all (if you have one of the latest cameras with anti-glare screens this may not be such a problem). Thus, the inability to judge a correct exposure on the camera's LCD is somewhat negated by auto bracketing since you should have the bracketed images to choose from when get home to view them on your calibrated monitor.
Sometimes you don't have time to chimp an image straight after taking it, or maybe you are photographing a short-lived moment that you might otherwise miss if you have to take a shot and then adjust your exposure because the camera's meter didn't get it right. Assuming your camera's meter wasn't way out on calculating the exposure, by bracketing your images you should still have a correctly exposed image to choose from.
Often (in the UK at least) a digital camera doesn't have enough dynamic range to record a scene as your eye sees it. In your photos you will normally find that either the sky is a blown out white while your subject (the countryside) is exposed properly, or the sky will be exposed properly and your subject will be very dark. If your digital camera decides to expose for the sky, by auto bracketing your images, you should end up with one dark image, one image exposed for the sky, and one image exposed for the countryside. Likewise, if your camera decides to expose for the countryside you should end up with one image exposed for the sky, one image exposed for the countryside, and one image over-exposed. Either way, you should end up with one image exposed for the sky and one image exposed for the countryside.
What you can then do is take the image exposed for the sky (as this will normally be the brightest part in any image), and then pull up the exposure in the shadows to get the countryside exposed correctly. Alternatively you may find you need or want to use Exposure Blending or HDR to merge the exposures into one 'correctly' exposed image.
UPDATE 12th December 2009: I have now posted a full tutorial on the process of Exposure blending to increase the dynamic range of an image.
The disadvantages of exposure bracketing are:
It uses up more storage since you record 3 images for every one image you take. With the cost of Memory cards and Hard drives today, this isn't too much of a problem though.
For action shots if you have Auto bracketing turned on, you'll probably end up with one underexposed shot, one correctly exposed shot, and one overexposed shot. Whereas with Auto bracketing off you can just fire off as many shots for as long as you hold the shutter down or until your camera's buffer is full.
Obviously, if you know you're going to be shooting action you wouldn't have Auto-bracketing turned on, but it can be a problem if you have Auto Bracketing on and then come across an action shot unexpectedly (normally some wildlife running away from you).
Why I like to use exposure bracketing
For me, I always use auto-bracketing when photographing landscapes for the three reasons I stated above - the camera LCD not being accurate, not having enough time to fine-tune exposure in the field, and the camera not having enough dynamic range to capture a whole scene as I see it most of the time.
For controlled lighting situations (e.g. anything using Flash) Auto Bracketing should not be needed.
Do you use Auto Bracketing, and why do you use or not use it? Any questions about using Auto Bracketing or anything else photography related?