Auto Exposure Bracketing

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Auto Exposure Bracketing (AEB) takes three exposures of the same scene: one exposure at the recommended setting, one 1/3 f-stop above the standard exposure, and one 1/3 f-stop below the standard exposure. This is the traditional technique for ensuring an acceptable exposure in scenes that have challenging and/or high-contrast lighting and in scenes that are difficult to set up again or that can't be reproduced.

But today, AEB is also used for image compositing where you take three different exposures of a high dynamic range scene, and composite them in an image-editing program to produce a final image that offers the best of highlight detail, midtone, and shadow detail. Using this technique, photographers can produce a final image that exceeds the range of the camera's sensor. Additionally, AEB is a mainstay of High-Dynamic Range (HDR) imaging, which merges three to seven or more bracketed exposures in Photoshop to create a 32-bit image with excellent rendering throughout the tonal range.

Note HDR imaging brackets exposures by shutter speed rather than by aperture to avoid slight shifts in focal length rendering.

On the 40D, the default exposure bracketing level is 1/3 stop, although you can reset the increment to 1/2 stop using C.Fn I-1. Exposure level increments AEB captures up to plus/minus 2 stops.

Here are some things that you should know about AEB:

♦ AEB is available in all Creative Zone modes except Manual, and it cannot be used with a Bulb exposure or with the built-in or external EX Speedlight or third-party flash unit.

♦ AEB settings remain in effect until you turn off the camera power switch or use the flash.

♦ In Continuous drive modes, you must press and hold the Shutter button to make all three bracketed exposures.

♦ In One-Shot AF mode, you must press the Shutter button three separate times to make the bracketed sequence.

♦ In the 2- or 10-second drive modes, the three bracketed shots are taken in succession after the timer interval has elapsed.

♦ The order of bracketed exposures begins with the standard exposure followed by decreased and increased exposures. You can change the order of bracketed exposures by setting C.Fn I-5.

♦ You can use AEB in combination with Exposure Compensation.

♦ If C.Fn III-7 is set for mirror lockup, and you're using AEB and Continuous drive mode, only one of the bracketed shots is taken at a time. You press the Shutter button once to lock up the mirror and again to make the exposure. In other words, a total of six presses of the Shutter button are required to complete three bracketed exposures. The exposure level indicator in the viewfinder flashes after each Shutter click until all bracketed images have been made.

To set AEB, follow these steps:

2. Press the Menu button, and, if necessary, press the Jump button to select the Shooting 2 (red) menu.

For details on setting Custom Functions, see Chapter 4.

For details on setting Custom Functions, see Chapter 4.

3. Turn the Quick Control dial to highlight AEB, and then press the Set button. The camera activates the Exposure Level Meter display.

4. Turn the Quick Control dial to the right to highlight the AEB amount that you want, and then press the Set button. As you turn the Quick Control dial to the right, the tick mark on the Exposure Level Meter separates into three marks indicating the bracketing amount. You can set up to plus/minus 2 stops of bracketing. The bracketing amount is also displayed on the LCD panel. AEB remains in effect until you turn it off. To turn off AEB, repeat Steps 1 through 4, but in Step 4, turn the Quick Control dial to the left until the three tick marks merge into a single tick mark at the center of the Exposure Level Meter.

With AEB set, when you press the Shutter button to take the exposures, the bracketing amount is displayed in the viewfinder and on the LCD panel Exposure level indicator as each shot is made.

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