What triggers a wireless slave flash?

kevsdad · Feb 28, 2013

I have a Canon T3i and will be purchasing a 430EXii soon. What actually triggers the off-camera flash (430EXii)? The light from the internal flash or is there a transmitter in the camera (T3i) that only works when the internal flash is open? Reason I ask is I have an old Vivitar 285 that I could use as the primary flash, but it must use the shoe, either on-camera of off by means of a cable, leaving the internal flash unusable as it will not open if the shoe is occupied. Will the light from the 285 trigger the off-camera 430 to fire or does the internal flash have to do that? Thanks.

tirediron · Feb 28, 2013

To use the built-in (TTL capable system) you will need to have your pop-up flash up; it communicates (via line-of-sight only) with the slave flashes using a series of virtually invisible micro-flash bursts which are "read" by the IR receiver on your slave flash. Some flashes have a built-in slave cell (that is, they will detect the flash of another speedlight and trigger themselves), and your version of the 285 may be one of them, in which case you could have it anywhere that it could "see" the light from your camera's pop-up flash and it would work, BUT you will be limited to manual flash only.

There are althernatives. You can get a optical slave "foot" which a speedlight will attach to, or you can use radio triggers.

Mully · Feb 28, 2013

RF units work very well and you can buy one for $30-40 on ebay

KmH · Feb 28, 2013

Infrared (IR) light from the camera is used to not only trigger the off-camera flash unit, the IR light can also communicate with the of camera flash.

Since it is light it is limited in range, particularly in direct sunlight because sunlight also includes interfering IR light, and pretty much to line-of-sight.

After market radio frequency (RF) triggers are very popular because they have a lot more range, can transmit through walls and around corners, and are not affected by sunlight.

Tiberius47 · Mar 1, 2013

It's a series of coded light pulses sent from the flash on the camera. It is NOT infra red light, since you can see the light pulsing from the camera's flash easily. It is visible white light. The pulses have encoded information that tells the slave flash when to fire and how strong to fire.

kevsdad · Mar 3, 2013

Thank you all for your help!

TCampbell · Mar 3, 2013

BTW, you can trigger a remote flash either wired or wireless. If wireless it can be visible light or IR or radio.

But in the case of YOUR camera ... the T3i's pop-up flash is using visible light (not radio or IR). You can add a radio module for more reliable triggering (line of sight isn't necessary) but you need both a radio transmitter and a radio receiver (there are versions that can support Canon E-TTL or you can get significantly less expensive radios that just do "manual" flash (you have to manually use the controls on the back of your 430EX II to tell it how much power to use when firing. It wont automatically be calculated for you like it is in E-TTL mode). You can also buy Canon's commander module (speedlite transmitter ST-E2 for IR) which would let you fire the flash without using the pop-up flash.

KmH · Mar 3, 2013

Tiberius47 said:
It's a series of coded light pulses sent from the flash on the camera. It is NOT infra red light, since you can see the light pulsing from the camera's flash easily. It is visible white light. The pulses have encoded information that tells the slave flash when to fire and how strong to fire.

The camera's flash is not capable of only emitting IR light. The invisible to humans IR flash communication signal is part of the visible light the flash unit creates.
The detector in the slave flash units only detects the IR portion of the light the camera's built-in flash unit creates.

Canon's ST-E2 also uses IR light, because that is the light frequency range the slave or eTTL capable flash units can detect. See the ST-E2 specifications: http://www.usa.canon.com/cusa/consu...up/speedlite_transmitter_st_e2#Specifications