This setting determines when the AF assist lamp is activated to emit a pulse of light that helps provide enough contrast for the EOS 40D to focus on a subject.
0: Emits. The AF assist light is emitted by the camera’s built-in flash whenever light levels are too low for accurate focusing using the ambient light.
1: Does not emit. The AF assist illumination is disabled. You might want to use this setting when shooting at concerts, weddings, or darkened locations where the light might prove distracting or discourteous.
2: Only external flash emits. The built-in AF assist light is disabled, but if a Canon EX dedicated flash unit is attached to the camera, its AF assist feature ...