If we are using an external third-party flash with no auto focus beam support, is there a setting to use the built-in auto focus beam with an external flash?
I'm using a Canon EOS 600D.
The 600D uses the built in pop-up flash for the focus assist light, the pop-up flash pops up too close the the external flash if one is connected - there are many external flashes and accessories that would be hit by the popup flash if it tried to open when they are connected.
So, as a safeguard the camera will not open the popup flash if there is something in the hot shoe.
And, since the auto focus assist light is actually the pop-up flash and you can't open it with an external flash attached there isn't any way to use the auto-focus assist with an external flash.