How A Slave Flash Works

Last Updated on February 3, 2023

What Is A Slave Flash?

portrait studio with lights

A slave flash is a remote, triggered flash that fires in sync with, or immediately following the firing of, another flash from a master unit. Slave flashes are often used to provide light in places where a photographer can’t get natural light because it isn’t bright enough or is not available. They can be mounted on an extension cord to be positioned outdoors and they will trigger off of the signal from a master unit inside.

For example, when shooting portrait photography, a flash unit usually wouldn’t be powerful enough to properly light an entire subject. A slave flash would be used to light the subject which is then brought into the composition using the master flash’s natural light.

Since a slave flash is typically triggered by a master unit, it can stay on constantly or only fire when the master itself fires, such as when using off-camera flash photography.

How Does It Work?

A slave unit only needs to be connected by an electrical conductor (e.g. an extension cord) to the master unit. Typically, the slave flash will trigger off of a radio transmission between its receiver and transmitter, meaning that the flash will fire in response to a signal sent from the master unit.

The signal sent from a master unit is typically set up in a serial digital protocol, allowing the slave unit to be triggered remotely using radio waves or infrared signals. This is a very different way for a flash to be triggered than the older-style manual hot shoe flashes.

For example, if a photographer wants to use a master flash with an off-camera radio trigger from a camera on a tripod, they would need to attach the slave Flash to the camera with an extension cord (unless it works via Bluetooth). They would then place their remote flash unit outside near the camera. The remote slave’s receiver would then receive the radio signal from the master unit and it will activate its transmitter and start firing its flash unit.

Basic slave flash units are designed to only trigger when they receive a radio signal from the master unit, but more advanced units can also trigger when they receive an infrared signal. An infrared strobe can be used for long-range flash photography where there is no direct line-of-sight to the remote flash unit.

How Is It Used?

There are many different ways a slave flash can be used. For example, if you’re shooting a wedding ceremony off of a tall roof and need some extra light, you could use a slave flash for that instead of attempting to get natural light in the area. This can be useful in poor weather or for shooting in low-light conditions.

Another example is when shooting a subject outdoors in a bright area and a photographer doesn’t want to use the flash to ruin the existing lighting conditions but fill in dark areas within the scene.

Using the built-in flash on most cameras, the light is often only useful in a small area and it can accidentally reflect off of the subject, causing problems. A slave flash would be placed outside of a studio or other location to avoid this problem.

Benefits Of Using A Slave Flash

Off-camera flash has a number of benefits above the standard on-camera flash. As mentioned above, one benefit of off-camera flash is that it allows a photographer to move the flash further away from the camera. That helps reduce lens flare and provides for softer light on the subject.

Another benefit of using a slave flash is that it can be used without firing the master unit. If you’re trying to, for example, take a picture of your child and you’re worried that if you turn on the master flash, he may become agitated and be distracted, it’s better to use an off-camera flash.

Difficulties Of Using A Slave Flash

A disadvantage of using slave flashes is that they typically don’t have dimmer control, which can lead to problems when the flash is used in the final photo. It’s also important to note that some slave flashes will not trigger correctly if they are used off camera with a wireless trigger system. This is common with low quality models.

You also don’t have control over the output on a slave flash. If you want to be able to adjust the output of a flash, you’ll need a slave unit that has manual control.


Slave flashes are a great way to avoid using the flash on your camera which can cause problems with lens flare and poor lighting. They’re popular among photographers who like to shoot off of light stands or outside when they don’t want to direct sunlight to ruin the scene or subject. When used correctly, a slave flash can provide even lighting over a large area from a flash off-camera. It can also fill in dark areas such as those faced when shooting portraits.