What is Auto Exposure Bracketing?
Auto exposure bracketing, or AEB, is a term that is used to signify a process where the camera automatically takes two or more exposures but with different exposure values. This method is generally used when the photographer isn’t quite sure what the correct exposure for a scene is. You would argue that the auto exposure mode could be a solution for this. True. But what if the camera gets it wrong? As you are aware a digital camera is designed so that it looks at every scene as if it is supposed to be 18% grey. That suggests that the metering system will over or under-expose, as required, to make the scene 18% grey, even if it means altering an image. That is why true black will appear grey in auto exposure mode as also true white.
Another reason, and a popular one at that, is when you wish to create HDR images. HDR stands for high dynamic range and that signifies an image that has a wider range of tones. This is something that is impossible to be captured in a single exposure. Photographers achieve this by combining two or more exposures in a photo editing software. Some digital cameras have built-in HDR mode, which too can achieve the same effect.
The problem of using auto exposure is that it is impossible to adjust a single exposure for a scene that has different light values at different areas. With three exposures, however, you can expose one for the highlights, one for the shadows and one for the midtones and then combine everything together. Essentially that is what HDR photos are all about.
Anyways, coming back to the question in hand, since auto exposure mode can get it wrong, and it does quite often, it is certainly not a solution, well not without taking some precautions. That precaution is AEB. So what does AEB does really? Well, as stated in the opening paragraph it takes two or more images automatically, but each time it uses a different exposure value.
This is how to use AEB:
- Press the AEB button and turn it on.
- Select the number of exposures you need. Two is the bare minimum.
- Select how much compensation to be used between each exposure.
- Turn on continuous shooting mode
- Compose the scene and press the shutter release to make the exposures.
Some points to note:
AEB will only work when you are shooting in Program mode, aperture priority and shutter priority mode. This will not work in programmed auto mode as well as manual mode. Since in manual mode the camera does not have any control over the exposure, the ultimate control being in your hand, you will have to adjust exposure yourself.
In aperture priority mode bracketing will change the aperture value. Let’s say you have metered a scene at f/4 (in aperture priority mode) at 1/500 of a second. You have also set AEB to take three shots at 1 stop compensation between. The first shot will be taken by the camera at f/4 at 1/500 of a second. The next shot will be taken at f/4 1/250 of a second and the final shot will be taken at f/4 at 1/1000 of a second.
This is of course considers that you had set the camera to take one exposure over and one under along with the perfectly exposed shot. You can change this around as well to take one exposure one stop under and another 2-stop under along with the perfectly exposed image and other combinations as well.
In shutter priority mode it is the aperture that gets changed around. This however can be a bit problematic when you cannot have the depth of field change. In programmed auto mode the will decide what to change, so you have absolutely no control over the depth of field. So, ideally AEB is best used in aperture priority mode.
AEB can be also used for changing ISO.