I think you need to use the sync speed of 1/60 for the F3, if you use any other shutter speed the shutter will not be wide open when the flash fires. A camera with a leaf shutter will syn at any speed with differnt guide number. A newer camera like the F4 or 5 has a higher flash sync not lower. Some older camera have a sync at 1/30. Or is the F3 a 1/30? Double exposure? Good luck.