The Canon EOS Rebel T3 Series Camera’s Flash
When you use the built-in flash with your Canon Rebel T3 or T3i, your camera automatically calculates the flash power needed to illuminate the subject. This process is sometimes referred to as flash metering. The T3 or T3i uses a flash-metering system that Canon calls E-TTL II. The E stands for evaluative, TTL stands for through the lens, and II refers to the fact that this system is an update to the first version of the system.
It isn't important that you remember what the initials stand for or even the flash system’s official name. What is helpful to keep in mind is how the system is designed to work.
First, you need to know that a flash can be used in two basic ways: as the primary light source or as a fill flash. When flash is the primary light source, both the subject and background are lit by the flash. In dim lighting, this typically results in a brightly lit subject and a dark background. This assumes that the background is far enough from the subject that it's beyond the reach of the flash, of course.
With fill flash, the background is exposed primarily by ambient light, and the flash adds a little extra illumination to the subject. Fill flash typically produces brighter backgrounds and, often, softer lighting of the subject because not as much flash power is needed. The downside is that if the ambient light is dim, you need a slow shutter speed to properly expose the image, and both the camera and the subject must remain still to avoid blurring.
The exposure mode you use (P, Tv, Av, M, or A-DEP) determines whether the flash operates as a fill flash or as the primary light source. The exposure mode also controls the extent to which the camera adjusts the aperture and shutter speed in response to the ambient light in the scene.
If the flash output in any mode isn’t to your liking, you can adjust it by using flash exposure compensation. You can also use exposure compensation to tweak the ambient exposure (the brightness) of your background. So you have multiple points of control: exposure compensation to manipulate the background brightness, and flash compensation and flash exposure lock to adjust the flash output.
Again, these guidelines apply to the camera’s built-in-flash. If you use certain Canon external flash units, you not only have more flash control but can also select a faster shutter speed than the built-in flash permits.