Originally Posted by smartelf
I am assuming that the flash fires for some interval of time(lets say 100ms). so if the scene is illuminated while the slit is moving, what is cussing the shadow?
The actual flash time for a strobe tube is generally 1ms or less. You need to use a shutter speed which allows both curtains to be fullly open, as Bob has explained. If your flash setting is still overexposing, you need to reduce the light either by moving farther back from the subject, using a higher f/stop, or filtering either the lens or the flash. The flash would be easier, as you would not have to worry about optical perfection.