Is there any way to fire 600ex speedlight off camera as a slave using pop-up camera flash?

[7D5D]

You can control it optically from a body such as a 7D (I think the 60D and newer Rebels can do this too).

Set up your flash for an optical slave then setup a compatiable camera (such as the 7D) to trigger the off-camera flash

Flash Control menu

Built-in flash func. setting

Wireless func.

- Top icon for built-in plus external (using power ratio)

- middle icon for external Speedlite only

- bottom icon for built-in plus external (no ratio, same power)

I did this briefly when I got my 600EX RT, then I bought the ST-E3-RT and I'm loving radio control flash and no more line-of-sight set ups. 

[Scott]

I have a T1i body.  I don't think it will work with this body.  Thank you anyway.  I was planning on upgradeing bodies in the near future, so now I know  what comes after that new 24-70.