Flash Triggers – Optical & Radio

When using a off camera flash, there are a few basic things we need to control as photographers. The placement, colour and quality of light are the 3 key elements which shape our shot. These are the elements where the artistry of photography and flash are applied.

But on a more basic technical level, we also need to control how the flash is triggered or fired as well as controlling the flash power.

Flash Triggers

In terms of triggers, there are generally two approaches. The first and simplest is that of a sync lead, which is a physical connection between the camera and the flash. This can be as simple as a wire which completes an electrical circuit causing the flash to fire and be as complex as a system which carries other signals which not only control the triggering but also the power of the flash output.

Wireless Triggers (Optical)

A second and more common approach is to use a wireless trigger, which in itself can be broken up into two categories: optical triggers and a radio triggers. Naturally enough, optical triggers use pulses of light to fire flashes. These can be both visible to the eye and also infrared, which the human eye can not see.

Like it’s wired equivalent, optical triggers can operate in two modes, where they can be used “simply” as a trigger signal for the off camera flashes, and also as a trigger which carries exposure and signalling controls. These controls can be used to set the output power of the flashes and also to do some fancy things like group flashes into separate clusters allowing you as a photographer to light different zones in a shot (i.e. Foreground, mid-ground & background) with different exposure values.

Optical Slave

Most flashes (including studio lights/monoblocks) can be operated in a slave mode. This is one step away from using a fixed sync lead. The power of the flash is fixed at the flash unit itself and using an infra-red light sensor on the flash unit it waits until it receives a pulse of light, which is the trigger signal. Once received the flash fires at which ever power the unit was set to. In this scenario, the trigger signal is also a contributing factor to the overall exposure and emanates from a flash that is actually used in lighting the scene, be that from a flash situated on the camera or another flash in the lighting setup, triggered either by sync lead or by a radio trigger.


A more complex optical system is that of Through The Lens triggering. This system uses a series of pre flashes between camera and off camera flashes to not only control the triggering but also to control the output power of the flashes. It’s usually applied only to flash guns as opposed to studio lights and indeed only between flash guns and cameras from the same manufacturer.

This is an automatic system where the combination of camera and flash determine the exposure and flash output power! Unfortunately this means the results can be both variable and indeed inconsistent. It really requires you to understand how and why your camera makes the exposure decisions it does, so that you can preempt these decisions and using exposure compensation, dial up and down the power of the flashes.

The pre-flashes occur prior to the exposure, so they do not in themselves effect the exposure of the scene. In order to use this setup, you require either a flash on your camera (which is effectively redundant apart from acting as a trigger) or a dedicated optical trigger. The pre-flashes are usually imperceptible to the human eye, but if you do want to see one, put your camera in manual mode, choose a shutter speed of about 1 second, put your camera into rear curtain sync and then fire the flash. You should now see two flashes, the first being the pre-flash and the second one a second later.

What’s Nice About TTL

Whilst I am not a fan of TTL, there are some very nice features and benefits that it offers (speaking from a Nikon point of view). The nicest has to be the ability to group flashes into different clusters, up to a maximum of 3 (A, B & C). The grouping is achieved by setting each flash individually at the unit itself. Once grouped, from the trigger flash, you can then control the power ratio between each group, up to a maximum of 6 stops (+/- 3 eV). If necessary, and it often is, you can also decide to manually set the power output of each group to a fixed power value which then offers you the full 9 stop range of the flash. This allows you to light different zones in a setup and control the lighting ratios between each zone.

TTL is also a plug and play system. You don’t need to meter (in fact you really can’t), you just point and click. The results are then tweaked by adjusting the exposure value in each group by means of compensation. In a way this can be considered a good thing but at the same time, it can also take a considerable number of exposures before you achieve the shot you want.

Unfortunately the system is very unpredictable and often inconsistent. The lighting and exposure can change from shot to shot. What you shoot, what you include in the frame, can change the light output from each group!! So if you’re a control freak, this system can often be very frustrating to use. If you’re happy to click away and to make adjustments on the fly, then perhaps it’s the way to go?

TTL & High Speed Sync

Cameras usually have a maximum shutter speed at which they can sync to the flash. However, it is possible to overcome this limitation by using a technique called high speed sync. Rather than one single strobe of light, high speed sync emits multiple strobes through the course of a single exposure following the gap between front and rear curtains as they traverse across the sensor. This allows the photographer to use much faster shutter speeds in combination with flash. Thankfully off camera TTL also affords this functionality to the photographer.

Disadvantage Of Optical Triggers

One significant disadvantage of optical triggers is that their range is reasonably limited and they often require line of sight between trigger and flashes. The range is very much determined by the shooting conditions. For example in a room with a little ambient light, the range can be considerable but as the level of ambient light creeps up, the range can decrease. A room with white or reflective walls can help increase the range where as shooting in the wide open or where the walls are dark, the range decreases again.

Wireless Trigger (Radio)

Radio triggers are an alternative wireless trigger, which use radio signals to trigger a flash. Predominantly these are simple dumb triggers that like a basic sync cable, complete an electrical circuit and fire the flash. The radio signal carries no signalling or control information. As such, the power of each flash unit has to be set at the unit itself.

More advanced dumb triggers (like the PocketWizards I use) have controls to specify frequency or channels, to avoid you interfering with other photographers around you, but lack the group functionality that optical TTL offers. So from the trigger, there is no way of controlling the output power of any of the flashes.

However, certain flash systems like the Elinchrom Quadra Ranger that I use, have smarter radio triggers that also allow you to control the power output and group of each flash. So the radio signal also carries control information. This is very similar functionality to the Creative Lighting System offered through Nikon flashes and offers a really nice advantage over the Pocket Wizards

Recently there have been some developments where radio triggers are starting to incorporate TTL control signals, but on the whole radio triggers simply fire the flashes they are connected to.

Advantages of Radio Triggers

Personally I prefer radio triggers over optical triggers, especially if they are configured into TTL mode. Primarily I prefer the control that the radio triggers offer. As the power of the flashes are set manually, I can meter and control my exposure precisely. What’s more, the flash part of the exposure does not change… Ever!! So unlike TTL, I do not have to deal with any inconsistencies or variance. The flash power is fixed and stays fixed.

For sure it’s a pain if I need to change the output power because it means I have to tweak each flash manually. But at least once I do, the value is locked down and fixed. Using the Elinchrom Skyport triggers, this is obviously made easier by offering the functionality of being able to control power (across different groups) from the trigger, i.e. from the camera.

Another massive advantage of radio triggers over optical triggers is that of range. First of all radio triggers do not rely on line of sight, so flashes and triggers can be hidden around corners, behind obstacles or indeed even in different rooms. Their range is significantly greater than their optical counterparts and the amount of ambient light has no effect (either way)

Mixing Trigger Systems

It is of course possible to mix the various trigger systems. For instance I could use a wireless trigger to fire one flash and then have other flashes set to optical slave mode, so that they fire once the main flash triggered by the radio signal fires. This is a cheaper alternative to buying multiple radio triggers but of course you are again dealing with the limitations of range and that the flashes must be in line of sight of the main flash.

One thing to watch for is when using optical slaves in combination with TTL systems. This often causes a problem because the pre-flash emitted during the metering/control stage actually fires the flashes (before the exposure itself). Although some systems, like the Elinchrom Quadras can “learn” the pre-flash and will then wait for the main flash.

Care to share your opinion?