Bulb

When I have done light painting using auto focus, I use the light source, or torch to get the focus holding down the cable release but not taking it all the way to open the shutter. When ready, release the shutter and paint away.
An alternative is to be close to the room light switch, with the room light on, focus, hold the release cable down,don't take it all the way, switch off the room light , press the release cable all the way and paint with your chosen light source.
Takes a bit of planing and faffing but it works for me.
I do hope I have explained my way of doing it clearly enough for you to get the idea.
HTH
An alternative is to be close to the room light switch, with the room light on, focus, hold the release cable down,don't take it all the way, switch off the room light , press the release cable all the way and paint with your chosen light source.
Takes a bit of planing and faffing but it works for me.
I do hope I have explained my way of doing it clearly enough for you to get the idea.
HTH