You can set the flash output from 1/128 power to 1/1 full power in 1/3 stop increments.
Use a hand-held flash meter to determine the required flash output to obtain a correct flash exposure.
Note: If the flash is set to manual mode, and the camera is set to AV, the shutter speed will blink and show the max synch speed. If the camera is set to TV mode, the minimum aperture (16,22,32 depending on lens) will flash. This is normal, as the camera has no idea how much flash the flash is going to output in this setup