Watch this video below by Canada-based photographer R.J. Hidson on long exposure and stroboscopic flash using a couple Nikon SB-800s and one SB-900. R.J. Hidson is a wedding and portrait photographer from Edmonton Alberta. There are 2 numbers that you have to remember with repeating flash: the first one is the number times the flash goes off, the second one is the speed of which is goes off. Those 2 will determine how long your exposure is going to be. Enjoy.