Using The Built In Flash; Depthof Field Preview; Brightness; Images - Canon EOS 550D Instruction Manual

Av : Changing the Depth of Field
Using the Built,in Flash
To obtain a correct flash exposure, the flash output will be set
automatically to match the set aperture (autoflash exposure). The
shutter speed will be set automatically between 11200 sec. - 30 sec. to
suit the scene's brightness.
In low light, the main subject is exposed with the automatic flash, and
the background is exposed with a slow shutter speed set automatically.
Both the subject and background look properly exposed (automatic
slow-speed flash sync). If you are handholding the camera, keep it
steady to prevent camera shake. Using a tripod is recommended.
If you do not want a slow shutter speed to be used, set the [_ Custom
Functions (CoFn)] menu's [Flash synco speed in Av mode] to [1: 1t
200-1160 sec. auto] or [2:1/200 sec. (fixed)] (p.192).
Depth of Field Preview_
Press the depth=of=field preview button
to stop down to the lens's current
aperture setting. You can then check the
depth of field (range of acceptable focus)
through the viewfinder.
The depth_0f-field effect Can be Clearly seen on the Live View image as you
change the aperture and press the depth-of-fie!d preview button (p,112),
82