A flash trigger is a device or system that tells a flash to fire when it is not attached to your camera. Triggers are what make off camera flash possible: they let you put a light anywhere, in a softbox beside your subject or behind them as a hair light, and still have it fire at exactly the moment you press the shutter.
Flash on top of the camera is convenient, but it lights everything from the front, which tends to look flat. Moving the flash off the camera instantly makes light more interesting and flattering. The only problem is that the flash needs a signal to fire. That signal is the job of a trigger. There are several kinds, from simple optical sensors to powerful radio systems, and choosing the right one makes off camera flash easy and reliable.
This guide explains in plain words the main types of triggers, how they work, what features matter, how to set up a radio trigger step by step, how to fix common problems, and the mistakes beginners make. There is a 10-minute exercise at the end and answers to common questions.
How a flash knows when to fire
When a flash sits in the camera’s hot shoe, the mount on top of the camera, metal contacts tell it when the shutter opens. It fires instantly. When the flash is somewhere else, that direct connection is gone. A trigger replaces it by sending a signal, by light, by radio or through a cable, to the flash at the right moment.
The transmitter is the part on or connected to the camera. The receiver is the part on or built into the flash. Many modern flashes have receivers built in, so you only need a transmitter on the camera.
Types of flash triggers
Sync cables
The oldest and simplest method is a cable running from the camera to the flash. It is very reliable and needs no batteries, but the cable limits how far the flash can go and is easy to trip over. Cables are still useful in studios and as a backup.
Optical slaves
An optical slave is a small light sensor on a flash. When it sees another flash fire, it fires too. You can trigger it with a small flash on the camera, or even the camera’s built in pop up flash at low power.
Pros: very cheap and often built into flashes. Cons: needs a clear line of sight to the triggering flash, works poorly in bright sun, and can be set off by other people’s flashes at events. Also, many cameras fire a quick pre flash to measure exposure, which can trigger simple slaves too early. Look for a setting that ignores the pre flash, sometimes called a digital or S2 mode.
Optical wireless systems
Many camera systems offer an optical wireless mode, where the camera’s flash or a master flash sends coded light pulses to tell remote flashes when and how strongly to fire. This can control power and even automatic flash from the camera.
Pros: often included with your camera and flash, with no extra purchase. Cons: needs line of sight, struggles outdoors in sunlight and has limited range.
Radio triggers
Radio triggers send the signal by radio waves. They work through walls, around corners, outdoors in bright sun and over long distances, often many tens of meters. Most modern radio systems also let you change each flash’s power and mode from the camera, which is a huge time saver.
Pros: reliable, long range, no line of sight needed, remote power control. Cons: costs more, uses batteries, and transmitters and flashes must use compatible systems.
For most beginners who want to use off camera flash, a radio trigger is the best choice.
Manual versus automatic triggering
Triggers come in two broad kinds.
Manual only triggers simply tell the flash to fire. You set the flash power yourself, on the flash or sometimes on the transmitter. These are simple and cheap.
Automatic capable triggers pass full information between camera and flash. They support TTL (through the lens) metering, where the camera measures the flash and sets power automatically, and high speed sync, which lets you use shutter speeds faster than the camera’s normal flash limit. These are more costly but very flexible.
Many photographers use manual flash most of the time because it is consistent, and TTL for fast moving situations like events. An automatic capable trigger gives you both options.
Features worth looking for
- Remote power control. Change each flash’s power from the camera without walking over to it.
- Groups. Assign flashes to groups, such as A, B and C, and set each group separately. For example, key light in group A, fill in B, hair light in C.
- Channels. Different channels let several photographers work in the same place without firing each other’s lights.
- Range. More range gives more freedom outdoors.
- Compatibility with your camera. Automatic features depend on your camera system.
- Built in receivers in flashes. Fewer separate parts and batteries.
- Clear display. A screen that shows every group’s power at a glance saves mistakes.
Setting up a radio trigger, step by step
- Turn everything off. Attach the transmitter to the camera’s hot shoe and tighten it.
- Prepare the flash. If the flash has a built in receiver, set it to wireless or slave receiver mode. If not, attach a separate receiver to its foot.
- Match channels. Set the transmitter and every flash to the same channel. If there is an ID or code setting, match that too.
- Assign groups. Put each flash in a group. With one flash, use group A.
- Turn on the flash first, then the transmitter, then the camera. Some systems are fussy about the order.
- Set flash mode and power on the transmitter. Start with manual mode at 1/8 power.
- Press the test button on the transmitter. The flash should fire. If not, check channels and groups.
- Set the camera to manual. A good starting point indoors is ISO 100 to 200, 1/160 second and f/5.6.
- Take a test shot and adjust flash power up or down until the exposure looks right.
Shutter speed and sync
Every camera has a maximum shutter speed it can use with normal flash, called the flash sync speed, often around 1/160 to 1/250 second. Go faster and a dark band appears across the photo, because the shutter is never fully open at once. Some triggers add a little delay, so the safe sync speed through a trigger may be slightly lower than with a flash on camera. If you see a dark band, slow the shutter a little.
If you need faster shutter speeds, for example to use a wide aperture in bright sun, use a trigger and flash that support high speed sync. See X sync for more on the technical side.
Mixing flash brands and systems
Radio systems are often specific to one maker or one family of products. A transmitter from one system usually cannot control flashes from another, except in basic manual firing mode with a separate receiver. Before buying, check that your trigger, your flashes and your camera all work together for the features you want. A simple way to stay out of trouble is to buy flashes and triggers from the same system, and choose a transmitter version made for your camera.
Speedlights versus studio strobes
Triggers work with both small and large flashes, with a few differences.
Speedlights are small, battery powered flashes that can sit on the camera or on a stand. Many have radio receivers built in, so you only need a transmitter. Their small size makes them easy to carry for location work.
Studio strobes are larger, more powerful flashes that plug into the wall or run on big batteries. Most modern strobes also have radio receivers built in. Older strobes often have a sync socket for a cable and a basic optical slave. You can still trigger them by radio by plugging a small receiver into the sync socket.
If you own both, a trigger system that controls both lets you use speedlights as hair or background lights and a strobe as the key, all from one transmitter.
Triggering more than one flash
Once you add a second or third flash, groups become essential. A common plan is:
- Group A: key light.
- Group B: fill light.
- Group C: hair or background light. See hair light and background light.
Use the same plan on every shoot, so you never have to remember which letter is which. You can put more than one flash in the same group if they should always match, such as two background lights on either side of a white backdrop.
Keeping the on camera flash useful
A trigger does not mean you must stop using flash on the camera. Some systems let you put a flash on the camera that acts as the transmitter and also fires itself. It can add a little fill from the front, or you can turn it to bounce off a ceiling. See bounce flash. Outdoors, a remote flash at low power makes excellent fill flash from the side, which looks more natural than fill straight from the camera.
Worked example: a one flash portrait in a park
Imagine you are photographing a friend in a park in the late afternoon. You have one flash with a built in radio receiver, a radio transmitter, a light stand and an umbrella.
- Place your friend with the sun behind them, so it lights their hair.
- Put the flash in the umbrella on a stand, about 45 degrees to the side and 1.5 meters away.
- Set transmitter and flash to the same channel and group A.
- Set the camera to manual: ISO 100, 1/200 second, f/5.6. Take a shot with the flash off. The background looks good, but the face is dark.
- Turn the flash on at 1/8 power. Take a shot. The face is a little bright.
- Lower the flash to 1/16 power from the transmitter. Now the face looks natural and balanced with the background.
Because the trigger is radio, it works even though the bright sun would have made optical triggering unreliable. For more on balancing flash with daylight, see flash and ambient light.
Worked example: a two light studio portrait
- Place a key light in a softbox (group A) and a hair light with a grid (group B).
- Set both to manual on the transmitter. Start with A at 1/4 and B at 1/8.
- Turn off group B from the transmitter and take a test shot to set the key light.
- Turn group B back on and adjust its power until the hair edge looks right.
- Make all further changes from the transmitter, without leaving the camera.
Group control turns lighting adjustments into a few button presses. For the lighting itself, see lighting ratios and the home photo studio guide.
Troubleshooting
- Flash does not fire. Check channels, IDs and groups match. Check batteries in transmitter and flash. Make sure the flash is in wireless receiver mode, not normal mode.
- Fires sometimes, not always. Low batteries, interference or distance. Move closer, change channel, replace batteries.
- Dark band across the photo. Shutter speed is above the sync speed. Slow it down.
- Exposure changes from shot to shot. You may be in TTL mode. Switch to manual for consistent results.
- Optical slave fires too early. It is reacting to the pre flash. Switch the slave to a mode that ignores pre flash, or set your camera’s flash to manual.
- Other people’s flashes fire your light. Change channel or ID.
- Flash goes to sleep. Many flashes power down after a while. Turn off sleep mode or tap the test button to wake it.
Caring for triggers and batteries
Triggers depend on batteries, and flat batteries are the most common cause of missed shots. Carry spares for the transmitter and every flash, and remove batteries from triggers you will not use for a while, to avoid leaks. Keep the hot shoe and contacts clean. Use a small bag or pouch so the transmitter is not damaged when packing. Label your flashes with their group letters, which saves confusion on busy shoots.
Why triggers change everything
A trigger costs less than most lenses, yet it changes your photography more than almost any lens could. Once your flash can sit anywhere, you can light from the side for shape, from behind for separation, or high above for a dramatic look. You start to think about light as something you place, not just something that happens. That shift is one of the most exciting steps in learning photography. For a full introduction to working this way, see off camera flash.
Common mistakes
- Mismatched channels or groups. The most common reason a flash will not fire.
- Buying incompatible gear. Check that trigger, flash and camera work together.
- Relying on optical triggering outdoors. Bright sun overwhelms optical sensors. Use radio.
- Shutter speed too fast. Stay at or below the sync speed unless using high speed sync.
- No spare batteries. Pack spares for every part.
- Leaving the flash in TTL by accident. Unexpected exposure changes. Use manual for consistency.
- Not labeling groups. It is easy to adjust the wrong light.
Try this: a 10-minute trigger test
- Minute 1: Put the transmitter on the camera and the flash on a stand or a table across the room.
- Minute 2: Match channel and group.
- Minute 3: Press the test button. Does the flash fire?
- Minute 4: Set the camera to manual: ISO 100, 1/160 second, f/5.6.
- Minute 5: Set the flash to 1/8 power and photograph an object.
- Minute 6: Change the power to 1/4 and 1/16 from the transmitter. Take a photo at each.
- Minute 7: Move the flash into another room, out of sight, and test again.
- Minute 8: Raise the shutter speed until a dark band appears. Note the speed.
- Minute 9: Return to a safe shutter speed.
- Minute 10: Write down your sync speed and favorite settings.
Frequently asked questions
What is a flash trigger?
A device that tells an off camera flash to fire when you press the shutter, using a cable, light or radio signal.
Radio or optical: which is better?
Radio is more reliable, works outdoors and around obstacles, and often lets you control power remotely. Optical is cheaper but needs line of sight.
Do I need TTL triggers?
Not to start. Manual triggers are simpler and consistent. TTL helps when the distance between subject and flash keeps changing.
Why does my flash not fire with the trigger?
Check channel, group, batteries and that the flash is in wireless receiver mode.
Can I use triggers from one system with flashes from another?
Usually only for basic manual firing with a separate receiver. Automatic features need matching systems.
How far do radio triggers work?
Often many tens of meters, depending on the system, batteries and surroundings.
Ready to move your flash off the camera? Start with the off camera flash guide.
Wondering why a dark band appears at fast shutter speeds? Read about flash sync speed.