This is the second episode of the series on how to trigger external flashes using an Infrared Commander.
An Infrared Commander is pretty much like a TV remote control. It uses the infrared to communicate with your external flashes which like with the built-in flash trigger; it needs to be set in slave mode.
The ratio is 8:1 on the left:
The ratio is 1:8 on the right:
Ratio 1-1:
You can also use it for High Sync Speed as long as your flash enables it. Same goes for TTL mode (automatic flash mode) as long as your flash does TTL.
It is a great piece of gear which I bought when I wanted to use HSS with my external flash. At that time I was not so much using it for studio work. Suffice to say that at the time I bought it, the Radio Trigger (which we cover in the next episode) are too expensive for amateurs.
The big drawback of this type of trigger is similar to the built-in flash acting as a trigger, they both rely on a line of sight so the flash can receive either the pre-flash light or the infrared signal.
It is worth noting this ST-e2 from Yongnuo is not compatible with the new Canon 5D III which dos not have a built-in flash and therefore requires a trigger of some kind.