Flash sync in av mode

with aperture Priority mode, your camera will set a shutter speed of up to 30 seconds to suit the brightness of the scene – even when you’re using flash. In low light, this means there’s a risk of recording a mixture of sharpness (from the flash) and blur (from the ambient exposure) While this can give creative results, you can reduce the blur by using the ‘Flash Synchronization Speed in Av Mode’ function to set a faster sync speed. You’ll find this option in the flash menu or the Custom Functions menu.