The camera is set to commander mode, A group, Channel 1 same as the flash. Yes, the flash is set to remote. It seems that everything is set up correctly. I tested the setting on Manual, Speed 1/320s and F/22. When the flash is on the camera, the picture is perfect. However, When flash is off camera, the picture is dark... despite we have the same settings.
I tested different aperture and shutter speed with flash off camera and then the picture is properly exposed. Do you know why?