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Flash: Wireless flash

The Speedlite Transmitter ST-E2 (attached to the top of the camera) can be used to fire one or more wireless-compatible Speedlites.

Canon’s wireless flash system means that you can fire two or more Speedlites together without any cables linking them.

Working with multiple Speedlites gives you creative control over your subject lighting. A main light can be used away from the camera, and other units can be used to provide backlighting or illuminate the background.

It has been possible to work with two or more flash units almost since the day they were invented. However, the problem has always been getting the exposure correct. Just synchronizing the flash outputs is not enough. You also need to control each output to give the correct amount of light for the subject.

Canon’s wireless flash does away with all this. You just set up the Speedlites and press the camera’s shutter release button. A series of pre-flashes are fired and the system automatically calculates the output required from each unit. It then fires all the units to give what should be a correctly exposed, well-balanced flash image.

The Canon wireless flash system is based on a master-slave relationship. The master unit is a flashgun, flash transmitter or built-in flash master fitted on or connected to the camera. The slave units are positioned on their own away from the camera. You can work with one master and one slave unit, or one master and between three and five slave groups. Each slave group can have one or more units. The system is very versatile, allowing almost unlimited control and creativity.

There are two types of wireless flash control within the Canon system. The newest and most versatile is the Radio Wireless system, introduced with the Speedlite 600EX-RT. The original system is an optical pulsed light system, found on previous Speedlite models.

Radio Wireless

The back of a Speedlite 600EX-RT flashgun showing it set to Radio Transmission Wireless Master setting.

Introduced with the EOS 5D Mark III, the Speedlite 600EX-RT flashgun features two different modes of multiple wireless flash control. As with previous Speedlite models, there is an Optical Slave function where it can function as either a wireless master or wireless slave. The second method is completely new – instead of an optical transmission, it uses 2.4GHz radio frequencies to control slave flashguns. This Radio Transmission (hence RT) removes two of the potential pitfalls with optical transmission; namely the limited range and the need for a line-of-sight connection between the master and slave. It also allows the use of up to five groups of slave Speedlites, rather than three, with up to three flashguns in each group for a total of 15 slaves.

Using the radio transmission, the range is extended to around 30m and slave flashguns can be hidden behind obstacles or subjects without fear that they may not fire.

As with optical transmission, when used in radio mode the Speedlite 600EX-RT can function as either a Radio Master or Radio Slave unit. If the Speedlite 600EX-RT is set as a slave unit it can be controlled by either another Speedlite 600EX-RT or the Speedlite Transmitter ST-E3-RT radio flash transmitter. So that you know when remote flashguns are ready to fire, the display on the back of the master flash will show a flash ready symbol when using the wireless radio transmission settings.

The back of the Speedlite Transmitter ST-E3-RT showing it set to Radio Wireless Master setting.

The Speedlite Transmitter ST-E3-RT is a radio transmitter launched with the Speedlite 600EX-RT. It is the same as a Speedlite 600EX-RT in functionality, but does not have a flash head so cannot produce any light.

When used with the EOS-1D X or EOS 5D Mark III DSLRs, the radio wireless settings allow more complex flash setups with the Group Firing Mode than an optical wireless system would allow. This mode allows you to set different flash modes for each of the five firing groups, so Group 1 and 2 may be E-TTL, Group 3 and 4 set to Manual and Group 5 using Auto external metering with the built-in thyristor. With earlier camera models, the maximum number of groups is reduced to three and the remote Speedlites in each group will switch to E-TTL automatically.

Optical pulsed-light wireless

In the Canon optical wireless system, flash settings and triggering pulses are sent by pulsed optical light from master units to slave units. This allows a control range of up to around 12m, but it is dependent on a line-of-sight connection between the master and slave unit.

The Speedlite 580EX (and 580EX II and 550EX] can be used as a master or a slave unit simply by moving a switch just above the shoe fitting.

There are several Canon flashguns – plus one Speedlite Transmitter and an Integrated Speedlite Transmitter – that can be used as master units in the Canon optical wireless flash system. These are the Speedlites 550EX, 580EX, 580EX II and 600EX, Macro Ring Lite MR-14EX, Macro Twin Lite MT-24EX and Speedlite Transmitter ST-E2. An Integrated Speedlite Transmitter is found on the EOS 7D, EOS 60D, EOS 600D and EOS 650D DSLR cameras.

Channels: There are four channel settings: both the master unit and the slave must be set to the same channel. This allows up to four photographers, each using a different channel, to work alongside each other without their flash units interfering with each other. Alternatively, one photographer can have four set-ups that can be controlled individually.

Controllable groups: The Speedlites 550EX, 580EX and 580EX II will control three groups of slave units. The ST-E2 will only control two groups. You can have any number of slave units in each group.

Master unit flash on/off: The Speedlites 550EX, 580EX and 580EX II can be used as a master unit with the main flash switched off. This allows it to control slave groups without adding to the illumination. ST-E2 always works in this way, as it does not have a flash unit.

Test firing: Pressing the test button on the master unit fires the slave units, allowing you to check that they are all operating correctly.

Modelling flash: Fires all the units with a rapid continuous flash, giving the impression of continuous lighting. This allows you to check the effect of the flash illumination before you make an exposure.

Speedlite Transmitter ST-E2

The Speedlite Transmitter ST-E2 mounts on the camera hotshoe. It provides a full range of control for one or more Speedlites, including groups and flash ratios.

The ST-E2 is an infrared transmitter that acts as a master flash unit. Although it emits no visible light when it fires, it can control two separate slave groups of flashguns.

Indoors, the ST-E2 can control flashguns up to 15 metres away. Outdoors this drops to 10 metres because of ambient light. Being infrared, ideally the unit should have a line-of-sight view to the flashguns, although if you’re working inside you may find that infrared bouncing from the ceiling and walls is enough to allow you to trigger the flashguns without needing precise line-of-sight.

The ST-E2 is attached to the camera hotshoe, facing forwards. It retains all E-TTL capability with E-TTL cameras and even allows you to use high-speed sync flash and flash exposure lock if your camera has an FEL button.

If you are using the transmitter to trigger more than one slave flash, you can set them up as two separate slave groups, group A and group B, and control the ratio between them. For simple backlighting, though, this is overkill. All you need the ST-E2 to do is trigger the slave flashgun. Simply put the ST-E2 in the camera’s hotshoe, set your flashgun to slave mode, and fire away. You don’t even have to set the power output as this is automatically done when you set the flash gun to slave.

The fine hairs on this flower are perfect for backlighting, especially when combined with a dark background. The light from behind has revealed their structure and texture. To light the flower head, the remote flash was placed on a tripod behind the subject, with a large diffusing screen to soften the shadows, and aimed towards the flower. It was triggered with an ST-E2 mounted on the camera. The great advantage of using a wireless flash system is that it can be used in the field without any difficulty.

Slave units

The Speedlites 550EX, 580EX and 580EX II can be used as master units by adjusting the settings on the flashgun. The slave units in the system include the models mentioned above as well as the 420EX, 430EX, 430EX II, 320EX and 270EX II. With the exception of the MR-14EX and MT-24EX Speedlites, which can only be master units, none of the other Speedlites in the Canon range are compatible with the wireless flash system.

Once you have chosen your slave unit, there are two other settings to make. First, the channel number must match that of the master unit or the slave will not respond. Second, you need to set the slave group ID. The term group is a little misleading, as you can have just one slave unit in a group. The group IDs are A, B and C.

A slave group can be thought of as one Speedlite divided into multiple flash units. All the slave units in the group fire their flash at the same strength to give a combined output. For example, if you have three Speedlites set to group A, all three will fire a pre-flash when they receive a signal from the master unit. The three pre-flashes are reflected back to the camera, where they are metered and stored. The camera then calculates the total output needed from group A. When the exposure is made, each of the three units in group A will provide one-third of the total output required. If a fourth unit is added to group A, each unit will provide one-quarter of the total output required.

Several Speedlites in one group can be useful when shooting close-ups at very small lens apertures. A single Speedlite might not be able to provide enough illumination at the small aperture, whereas a group will combine to give, in effect, a more powerful gun.

One unexpected benefit of using several Speedlites together in a group is that the reduced output of each gun means a shorter flash duration. This can be very useful if you want to ‘freeze’ a fast moving close subject, such as a milk droplet as it hits a surface.

Setting the group ID is only important if the flash ratio control on the master unit is ON. If the flash ratio control is OFF, all the Speedlites will fire with the same flash output, irrespective of their group setting. Each Speedlite will contribute an equal share of the total flash output.

Flash ratio control

You can use the master unit to set different flash outputs for slave groups A and B. The A:B flash ratio can be set from 8:1 to 1:8 in 0.5 step increments (a total of 13 settings). The total flash output of groups A and B is set to provide correct exposure. The pre-flash is fired by the slave groups in sequence. The readings obtained are used to set each slave group’s flash output in accordance with the flash ratio.

For example, if the A:B ratio is set to 2:1, the flash output of slave group B will be one stop lower than that of slave group A, but together they give a well-exposed image. Unlike studio flash, you do not need to move some flash units further away from the subject than others to produce creative effects − the Canon wireless flash system controls the flash output for you.

If you have three flash groups, a pre-flash is fired by each group in turn. Slave groups A and B can be set to a flash ratio, as above. Slave group C is set to give correct exposure independently of groups A and B. This means that the Speedlite(s) in group C must be used with care. If aimed at the background or used from behind the subject for rim or accent lighting, the exposure may be correct. However, if group C units are aimed at the subject, then the additional light will give overexposure. This can be avoided by setting minus flash exposure compensation for slave group C. You may also need to use flash exposure compensation if the background or accent lighting needs to be subdued.

Flash ratio control is only possible between slave groups A and B. Flash ratios A:C or B:C are not possible.

How it works

As soon as the shutter button is pressed, the camera takes an ambient light meter reading. This is stored in the memory. Next, the master unit transmits a signal to slave unit A (or all slave units set to group A if there is more than one). This triggers a pre-flash from the unit (or units). This flash is metered and stored in the memory. This sequence is repeated in turn for the unit or units in group B, and then in group C.

The master unit knows if you are not using units set to group B or C because it will not receive back any pre-flash data.

The memory now has readings for the ambient light, plus the pre-flashes from groups A, B and C. With this information, the camera can calculate the output needed from each of the flash units for a balanced exposure. This information is transmitted to the units, and they all fire in synchronisation with the opening of the camera shutter.


This diagram shows how the Canon wireless flash system operates. Click on the diagram for a larger view.

A Speedlite master flash unit communicates with the slave units using visible light (infrared radiation with the Speedlite Transmitter ST-E2). The information that can be sent is:

Channel: 1, 2, 3 or 4

Flash mode: Number of groups (1, 2 or 3); E-TTL/M/Multi; pre-flash/main flash; normal/FP flash

Flash output: 1 to 1/128 power

Stroboscopic flash: firing frequency and number of flashes

The information is coded and compressed into a 2 to 5 byte signal. This is transmitted as a 10KHz high-speed pulse. Everything happens at great speed. Both the pre-flash and the main flash fire so close together that most people will only see a single flash.

One advantage of the coded optical pulse is that the slave units will not fire if they receive a single flash from other photographers who are also using flash guns in the same area.

The time it takes for the synchronised flash to fire after the shutter button is fully depressed is between 80ms and 130ms, depending on the EOS camera model and the number of slave units.

Outdoors, the receiver on the slave unit must be in line of sight of the master flash. Indoors, the optical pulse will usually reach the slave unit after bouncing of a light-toned wall or ceiling. Outdoors, the range of the optical pulse from the Speedlite 550EX is about 8 to 10 metres. The range of the Speedlite Transmitter ST-E2 is less − up to about 5m with good line of sight. Indoors, the range is a little more, suitable for all but the largest rooms.

Wireless compatibility

Canon wireless flash control can be used with most EOS camera bodies, but there are restrictions with earlier models.

The EOS 750 and 850 cannot be used at all.

Other Type B cameras can be used with wireless manual flash. Here, you preset the slave unit output from the master unit, using a hand-held flash meter to determine the correct exposure. The slave output can be uniform or different for groups A, B and C. Type B cameras are the EOS-1, 1N, 10, 100, 1000 series, 3000, 5, 500, 5000, 600, 620, 650, 700 and RT.

Type A cameras offer E-TTL flash metering and can be used with Canon wireless flash control. Type A cameras are the EOS-1V, 3, 30, 33, 300, 300V, 300X, 3000N, 3000V, 50, 50E, 500N, IX, IX7, 1D series, 1Ds series, D30 and all later digital models.

However, only the more recent Type A cameras support wireless flash ratio control. These models are the EOS-1V, 3, 30V, 33V, and the digital cameras listed above.

Wireless flash control

With the introduction of the EOS-1D Mark III it became possible to control wireless flashguns from the camera menu system rather than having to make changes to settings on the flashguns themselves. This speeds up control and avoids the need to go to each flashgun in a multiple wireless set-up to change the settings. It also provides a clear interface for making settings.

Although it is possible to use wireless flash in full E-TTL II flash mode, many photographers choose to set flash powers manually for each of the three possible groups of flashguns – A, B and C. This can also be achieved from the camera menu system.


The wireless setting screen for control of remote flashguns in a wireless setup. This screen is only available when using a 580EX II on-camera to control remote flashguns.

The camera menu system for flash control is only functional when using Mark II EX Speedlites such as the 580EX II and 430EX II. With the original 580EX, you have to make the settings on the back of the master flashgun or on each of the individual flashguns in their respective groups.

The cameras which feature on-camera flash control settings are the EOS-1D Mark III, EOS-1Ds Mark III, EOS 5D Mark II, EOS 50D and EOS 40D.

The EOS 7D was the first EOS DSLR body that was able to control Speedlites wirelessly without the use of a master Speedlite or IR transmitter. This wireless control ability is now also offered in the EOS 60D, EOS 600D and EOS 650D.

Easy Wireless

Introduced with the EOS 600D DSLR, the Easy Wireless feature makes it simple for new users to begin to make use of the wireless flash abilities of the EOS system without having to understand a lot about wireless flash to get good results.

In operation, Easy Wireless requires nothing more than ensuring your flash is set to be a slave unit on the same channel as the camera and the Easy Wireless function is selected in the camera menu.

In Easy Wireless mode, you don’t have control of the ratio between groups. Instead, the flashgun groups will be treated as one big pool of light, mimicking the A+B+C group mode available in the custom wireless settings.