With the camera, you can set the EOS-dedicated Speedlite's flash exposure compensation amount up to +/-3 stops in 1/3-stop increments
While pressing the button, turn the dial to set the desired flash exposure compensation amount
On the LCD panel, the plus side of the scale indicates underexposure and the minus side indicates underexposure. The sample display shows +2/3 stop flash exposure compensation
Take the picture
Press the shutter button halfway to display the flash exposure compensation amount on the viewfinder's exposure level scale.
Press the button to display the flash exposure compensation amount on the LCD panel
To cancel flash exposure compensation set the flash exposure compensation amount to "0"
button, turn the 
dial to set the desired flash exposure compensation amount