Light painting

This is Metate Arch in the Devil's Garden near Escalante, Utah. Last night I brought my photo tour group here for a session in light painting. During a 15 second exposure, I illuminated the arch with a flash light. After two or three test shots, I determined the settings should be f/8 and 100 ISO. The exposure is impossible for the camera's meter to read. There are too many variables such as intensity of the light, the distance from the flashlight to the arch, the f/stop, and the ISO, plus meters don't do well in darkness. We were shooting between 8:30pm and 9:15pm, but the Milky Way was going to be in the right position at 4am. We didn't wait for that; instead, I took a shot of the Milky Way from a previous evening's shoot and composited it behind the arch.