What is Repeating Flash?
Many accessory flashes have a repeating flash mode that can be set on the flash (Speedlight or Speedlite) while it is attached to the camera, or set through a flash controller when the flash is off the camera.
Repeating flash mode causes the flash to fire several times during a single exposure. It is also called strobe lighting. If used in a low ambient light situation, it will have the effect of creating multiple frozen images of a moving subject in the picture. Since flash fires very quickly, we will not see motion blur in the image.
How is it Set?
Use your camera in manual mode to set the aperture, ISO, and shutter speed for the shot. If there is some ambient light, you’ll want to use a smaller aperture and ISO to cut down ambient light. However, this also reduces the effective power of the flash, so you may need to increase the flash power used.
On the flash, or flash controller, select these three settings:
- Flash power. This controls the power of the flash for the individual pulses of light. It can be set from full power (1/1) to 1/128 power. You cannot use full power if you use more than one pulse of flash during the exposure. Logically, you would need ½ power for two pulses, ¼ power for four pulses etc., but I have a hunch you should leave some power to spare, so I might to choose a lower flash setting. I find my Nikon Speedlight automatically limits the power I can set, depending on the other two settings below.
- Number of times. This setting represents how many times you want the flash to fire during the exposure. For example, 5 times will cause the flash to pulse five times during the exposure at the flash power setting.
- This setting sets the number of times per second you want the flash to fire during the exposure.
What Shutter Speed to Use?
The shutter speed needs to be slow enough to allow the total number of flashes to fire within the time the shutter is open. To get a rough estimate of this, multiply the number of times by the hertz.
5 times/5 hertz = 1 sec. The flash will fire 5 times within a second, so you need at least a 1 second shutter speed.
10 times/5 hertz = 2 sec. The flash will fire 10 times in a two second period, so you need at least a 2 second shutter speed to capture every flash.
If the shutter speed is too fast you will not see all the flashes in your picture. If it is too long, you will see all the flashes, but you will also be capturing more ambient light in the shot.
In the above experiment, I suspended a ballpoint pen from a piece of cotton thread and set it to swing in a low-lit room. My camera was set to ISO 100, f/5.0. , 1 second. My flash was set to repeat 5 times at 5 hertz. Flash power 1/16.