When using flash, in
aperture priority shooting mode, the
Canon system assigns the flash as fill
light.
Therefore sets a slow
shutter speed to achieve a correct
exposure (as per the cameras
meter).
When flash is used in Tv (time value/
shutter priority) mode or manual, the
canon system assumes that the flash is the main source of
light and adjusts appropriatley.
The faster you set your
shutter speed the darker the background is going to be
The higher your
ISO, and the slower your
shutter speed, the more ambient
light will be recorded by the sensor, so you need to decide how much of that ambient
light you want in your image and set the camera accordingly