slow sync- technique page

Slow sync is when the camera has a longer shutter speed as well as firing the flash.
I did this in the studio so you have to do it against a black ground.
Use the red head as the constant light  but have polystyrene boards blocking any light falling on the background.
exposure reading for that light, then add in the flash for that apiture and use rear curtain sync so the flash goes off at the end.