While it's very convenient to be able to pop-up the built-in flash if there is one in your camera or slip a Speedlite into your EOS camera’s hot-shoe, you'll often get more attractive results by using off-camera flash. That's where Canon's wireless flash system can help by enabling you to fire one or more Speedlites simultaneously without any cables connecting them.

Working with multiple Speedlites gives you fantastic creative control over your lighting. For example, with the main light away from the camera you can provide flattering side-lighting to reveal your subject's shape while other units fill in the shadows, add 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 synchronising the flash outputs is not enough – you also need to control each output independently to give the correct amount of light for the subject.

Canon's wireless flash system can handle this for you, making it easy to create the images you want. You just set up the Speedlites and press the camera's shutter release button. A series of pre-flashes will be fired and the system automatically calculates the output required from each flash unit. It then fires all the units simultaneously to give what should be a correctly-exposed, well-balanced flash-lit image.

The Canon wireless flash system is based on a transmitter-receiver relationship, with the transmitter unit controlling the receiver units. The transmitter unit can be a built-in flash or an external Speedlite flash or flash transmitter fitted on or connected to the camera. The receiver flash units are positioned away from the camera and directed towards whatever you want them to illuminate. You can work with one transmitter and one receiver unit, or one transmitter and a number of receiver groups (up to five groups, depending on which wireless control system you're using). Each receiver group can have one or more receiver flash units. This means that the system is very versatile, allowing almost unlimited control and creativity.

There are two types of wireless flash control within the Canon system. The more versatile is the Radio Wireless system, which was introduced with the Speedlite 600EX-RT in 2012. The original system is an optical pulsed light system, found in previous Speedlite models and supported in all current models (although a couple of macro light units can work only as transmitters with the optical system, not as receivers). The optical system has some limitations (shorter wireless range, for example, and issues working in bright sunlight) but also some advantages (such as the potential to use more receiver flash units than the Radio Wireless system supports).

There are currently five Canon Speedlites that use the Canon Radio Wireless system: the Speedlite EL-1, Speedlite 600EX-RT, 600EX II-RT, 430EX III-RT and Macro Twin Lite MT-26EX-RT. The RT stands for "Radio Transmission".

The Radio Wireless system uses the 2.4GHz radio frequency to control receiver flashguns. This Radio Transmission removes two of the potential pitfalls with optical transmission, namely the limited range (especially outdoors) and the need for a line-of-sight connection between the transmitter and receiver. Using the RT system, the range is extended to around 30 metres and the receiver flashguns can be hidden behind obstacles or subjects without fear that they might not fire. That's useful if you want to light your subject from behind or light the background with a Speedlite hidden behind the subject.

The RT system also allows the use of up to five groups of receiver Speedlites (as compared to three with the optical system), with up to three flashguns in each group, for a total of 15 receiver flash units.

In radio mode, the Speedlite EL-1, 600EX II-RT and 430EX III-RT can function as either a radio transmitter or a radio receiver unit, while the Macro Twin Lite MT-26EX-RT, which is used mounted on the lens, can act only as a transmitter. If the Speedlite EL-1, 600EX II-RT or 430EX III-RT is set as a receiver unit, it can be controlled by any RT system transmitter, including the Speedlite Transmitter ST-E3-RT radio flash transmitter.

The Speedlite Transmitter ST-E3-RT mounts on the camera hot-shoe and has the same radio transmitter functionality as the Speedlite 600EX II-RT, but it doesn't have a flash head, so it cannot produce any light itself. It's ideal when you don't want front-on flash from a unit on the camera, and it's smaller than a flash unit, so you can work without the extra bulk and weight on the camera.

When you're using the wireless radio transmission settings, the display on the back of the transmitter unit shows a flash ready symbol so that you know when remote flashguns are ready to fire.

It's essential that the transmitter and receiver units are set to the same channel and ID so they can communicate.

In the Radio Wireless system there are 15 channels available, and you can either set a specific number from 1 to 15 or use the Auto option to scan for the channel with the best signal and use that. The ID is a four-digit number from 0000 to 9999 that you set to be the same on all the units. Once a connection has been established between the transmitter and receiver units, the Link lamp turns green.

Using a specific transmission channel and wireless ID avoids interference from other photographers who also happen to be using radio flash.

A receiver group can be thought of as one Speedlite divided into multiple flash units (up to three per group with the Canon Radio Wireless system). All the receiver units in the group fire their flash at the same strength to give a combined output. For example, if you have two Speedlites set to group A, both will fire a pre-flash when they receive a signal from the transmitter unit. The two pre-flashes are reflected back to the camera, where they are metered and remembered. The camera then calculates the total output needed from group A. When the exposure is made, each of the two units in group A will provide half of the total output required. If there are three units in group A, each unit will provide one-third of the total output required. This gives you the potential of greater output than a single flash gun could provide, or just faster recovery time if full power isn't needed.

If you're using a Speedlite flash as the transmitter, it will itself normally fire in Group A. However, you may prefer to set it to not fire, so that it operates just like the Speedlite Transmitter ST-E3-RT and doesn't contribute to the illumination of the subject.

Having 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.

Another benefit of using several Speedlites together in a group can be that the output of each gun is reduced, which 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.

When the transmitter and receiver units are both set to E-TTL (or E-TTL II), they operate in Fully Automatic Wireless Flash mode. However, you can control the receiver flash exposure compensation, flash exposure bracketing, flash exposure lock and high-speed sync, or select manual flash and stroboscopic flash via the transmitter unit.

With the transmitter and receiver units set up, as soon as the camera's shutter button is pressed, the camera takes an ambient light meter reading and stores this in memory. Next, the transmitter unit sends a signal to receiver unit A (or all receiver units set to group A if there is more than one). This triggers a pre-flash from the unit (or units in that group). This flash is metered, and the reading stored in memory. This sequence is repeated in turn for the unit or units in group B and then group C, if any.

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

You can use the transmitter unit to preset different flash outputs for two receiver groups, A and B. The A:B flash ratio can be set from 8:1 to 1:8 (which corresponds to a range from 3:1 to 1:3 when converted to number of stops of exposure) in 0.5-stop increments – a total of 13 settings – with 1:1 giving equal output from the two groups. The increment can be changed to 0.3 stops if you prefer. If you're using a third receiver group as well, you can set the flash ratio A:B and control group C separately using flash exposure compensation. As the RT system supports up to five groups, the same principle applies when you're using a group D or E in addition.

If you're using two receiver groups, the total flash output of groups A and B is set to provide correct exposure. The pre-flash is fired by the receiver groups in sequence, as mentioned. The readings obtained are used to set each receiver 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 receiver group B will be one stop lower than that of receiver group A, but together they give a well-exposed image. Unlike when you're using 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. Receiver groups A and B can be set to a flash ratio, as above. Receiver group C is set to give correct exposure independently of groups A and B. This means that the Speedlite (or Speedlites) in group C must be used with care. If group C is 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 result in overexposure. This can be avoided by setting minus flash exposure compensation for receiver group C. You may also need to use flash exposure compensation if the background or accent lighting needs to be subdued.

If the transmitter unit and the receivers are set to Manual rather than E-TTL, it's possible to set a different flash output for each receiver group, giving you greater creative control over your images. After assigning each receiver to a firing group, you need to set the output of each group via the transmitter unit.

The ability to control the output of a receiver from the transmitter unit is especially helpful when the receiver Speedlites are awkward to access.

With the Radio Wireless system, using Group Firing (Gr) Mode enables you to set a different flash mode for each group, with up to five groups (A, B, C, D and E). The available modes are E-TTL II/E-TTL autoflash (with or without flash exposure compensation), Manual, or Auto external flash metering. This could be useful, for example, if you want to use E-TTL II flash on a portrait subject's face (group A and B, for example) but take manual control over the hair light (group C) and background illumination (group D).

In the Canon optical wireless system, flash settings and triggering pulses are sent by pulsed light (usually infrared) from the transmitter unit to the receiver units. This allows a control range of up to around 15 metres, but it is dependent on a line-of-sight connection between the transmitter and receiver units.

There are several Canon flashguns and one Speedlite Transmitter that can be used as transmitter units in the Canon optical wireless flash system. These are the 600EX II-RT, Speedlite EL-1, Speedlite EL-100, Macro Ring Lite MR-14EX II, Macro Twin Lite MT-26EX-RT and the Speedlite Transmitter ST-E2 (more about this later). An Integrated Speedlite Transmitter is also found in various EOS cameras with built-in flashes, such as the EOS 90D, EOS 7D Mark II, EOS 77D, EOS 800D and EOS 850D, enabling them to emit infrared signals to control remote Speedlites in a similar way to the Speedlite Transmitter ST-E2 but without the need for an additional transmitter unit.

As with the Radio Wireless system, the transmitter and receiver units need to be set to a specific channel to communicate. In the optical system there are four channels available – which means that, provided they each use a different channel, up to four photographers can work in the same location without their flash units interfering with each other. Alternatively, one photographer can have four separate set-ups that can be controlled individually.

The Speedlite EL-1, 600EX II-RT, EL-100, Macro Ring Lite MR-14EX II and Macro Twin Lite MT-26EX-RT can control up to three groups of receiver units using the optical wireless system. The same applies to cameras with an integrated Speedlite transmitter built-in. The Speedlite Transmitter ST-E2 will control only two groups. You can have any number of receiver units in each group.

As with the Radio Wireless system, the transmitter unit can be used with its own flash switched off. This allows it to control receiver groups without adding to the illumination. The Speedlite Transmitter ST-E2 always works in this way, as it does not have a flash itself.

The Speedlite Transmitter ST-E2 mounts on the camera hot-shoe and provides a full range of control for one or more Speedlites, including control of groups and flash ratios. It can control two separate receiver groups of flashguns.

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

The ST-E2 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 receiver flash, you can set them up as two separate receiver groups, A and B, and control the ratio between them (as explained above). For simple off-camera lighting, though, simply put the ST-E2 in the camera's hot-shoe, set your flash unit or units to receiver mode, and fire away. You don't even have to set the power output – this is done automatically when you set the flash unit to receiver.

All of the current Speedlite range can be used as optical wireless receiver units apart from the Macro Ring Lite MR-14EX II and Macro Twin Lite MT-26EX-RT, which can only be transmitter units in the optical wireless system.

Once you have chosen your receiver unit, there are two other settings to select. First, as noted earlier, the channel number (1-4) must match that of the transmitter unit or the receiver will not respond. Second, you need to set the receiver group ID (A, B or C). This applies even if you have a "group" with just one receiver unit in it. However, in the optical wireless system, setting the group ID is important only if the Flash Ratio Control on the transmitter 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 that the camera has calculated is required.

Introduced with the EOS 600D and now found in a number of models like the EOS 7D Mark II and EOS 850D, the Easy Wireless feature makes it simple for new users to begin to make use of Canon's wireless flash system without having to understand a lot about wireless flash to get good results.

Easy Wireless uses the flash built into the camera as a transmitter. It requires nothing more than ensuring your external Speedlite is set to be a receiver unit on the same channel as the camera and the Easy Wireless function is selected in the camera menu.

Easy Wireless mode uses Canon's optical wireless system, so you can use up to three groups of external flash units, but 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.