hi! i;ve a 18-55 kit lens with canon 500d + an external flash. I try to shoot in a low light condition by attaching an external speedlite flash. in P mode i notice that the shutter speed will increase to about 60, but for other mode like AV. I'm not able to increase the shutter speed by increasing the aperture (eg. the shutter speed is always below 1/20). Those settings are still the same even when the flash light is attached. How does an external flash help when i shoot in av mode?