You might never notice it, but when you press the shutter button on your Canon Rebel T3 or T3i to take a picture with flash enabled, the camera emits a brief preflash before the actual flash. This preflash is used to determine the proper flash power needed to expose the image.
Occasionally, the information that the camera collects from the preflash can be off-target because of the assumptions the system makes about what area of the frame is likely to contain your subject. To address this problem, your camera has a feature called Flash Exposure Lock, or FE Lock. This tool enables you to set the flash power based on only the center of the frame.
Unfortunately, FE Lock isn't available in Live View mode. If you want to use this feature, you must abandon Live View and use the viewfinder to frame your images.
Follow these steps to use FE Lock:
Frame your photo so that your subject falls under the center autofocus point.
You want your subject smack in the middle of the frame. You can reframe the shot after locking the flash exposure, if you want.
Press the shutter button halfway.
The camera meters the light in the scene. If you’re using autofocusing, focus is set on your subject, and the green focus confirmation dot appears in the viewfinder. You can now lift your finger off the shutter button, if you want.
While the subject is still under the center autofocus point, press and release the AE Lock button.
The camera emits the preflash, and the letters FEL display for a second in the viewfinder. (FEL stands for flash exposure lock.)
If needed, reestablish focus on your subject.
In autofocus mode, press and hold the shutter button halfway. (Take this step only if you released the shutter button after Step 2.) In manual focus mode, twist the focusing ring on the lens to establish focus.
Reframe the image to the composition you want.
While you do, keep the shutter button pressed halfway to maintain focus if you’re using autofocusing.
Press the shutter button the rest of the way to take the picture.
The image is captured using the flash output setting you established in Step 3.
Flash exposure lock is also helpful when you’re shooting portraits. The preflash sometimes causes people to blink, which means that with normal flash shooting, in which the actual flash and exposure occur immediately after the preflash, their eyes are closed at the exact moment of the exposure. With flash exposure lock, you can fire the preflash and then wait a second or two for the subject’s eyes to recover before you take the actual picture.
Better yet, the flash exposure setting remains in force for about 16 seconds, meaning that you can shoot a series of images using the same flash setting without firing another preflash at all.
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Source:http://www.dummies.com/how-to/content/lock-the-flash-exposure-on-a-canon-eos-rebel-t3-se.html
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