Fire a canon 55ex off camera using another flash unit?

[ShutteringFocus]

Can I fire my 550 ex speedlight off camera by using the light from another flash unit?

Or is using the speedlight's wireless sensor system the only way to fire the 550 off camera?

What I want to do is have my 550 fire as a fill flash and I want it to fire by reacting to the flash from the other units. Is this possible?

[Nortelbert]

Should be OK, I used to do it with my 420 and 580 acting as the master.

[PacAce]

Can I fire my 550 ex speedlight off camera by using the light from another flash unit?

Or is using the speedlight's wireless sensor system the only way to fire the 550 off camera?

What I want to do is have my 550 fire as a fill flash and I want it to fire by reacting to the flash from the other units. Is this possible?

If the flash that triggers the 550EX is not another 550EX or a 580EX (I or II), then you'll need to connect teh 550EX to an optical slave device if you want it to be triggered by another flash (other than the built-in flash).

[hawk911]

right- the 550ex does not have an optical slave sensor in it. You either have to use another ex model that can act as master, and the 550ex as an infrared slave. Or as said, buy an optical slave hotshoe adapter.