Philip, wihtout a problem. This feature is called "optical slave" and many 
flash units have it.

What you loose in such situation is [E-]TTL. So, put your 550EX in Manual slave 
mode and set the apperture and power on it.

Then put it off camera (on a tripod or something) and put your flash on the 10D 
up.

Now, you can block the flash on the camera with your hand, piece of cardboard 
or something and only the 550EX will have efect on the picture.

OR, you can use both to light the scene (in which case turn on some FEC on the 
on-camera flash for bests results).

You can get strikingly good results with such combinations. If you flash is 
close to a wall, you can turn 550EX's head into the wall, dial in more power 
and have a "softbox" sort of. Great results indeed.

Regards,
  Miha.

> -----Original Message-----
> From: Phillip Moore
> 
> I recently took a workshop from my local photo club where the 
> workshop 
> instructor
> suggested he used a really old flash to light his backgrounds.  It 
> apparently was
> activated just by the flash light of his main flash and not by any 
> fancy wireless activity.
> 
> Right now I only have a single 550EX to use with my 10D.  I was 
> thinking that I might be able to get some reasonable results  
> by using 
> the on camera flash to activate the 550EX off camera on its 
> own tripod. 
>   I have the hot shoe cord but it doesn't reach very far.
> 
> Anyone know if the 550EX can be made to activate itself when it  
> detects other flashes going off? I can't find anything about that in 
> the manual.
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