ISO

The flash(Left) caused my partner to be visible but completely dulled the lights behind her.
The ISO 5000 (center) allowed the light to be seen but it's pixelated a bit, grained so to speak. However, you can see both my partner and the lights clearly.
The ISO 50 (right) is dark and hard to see. However, unlike the ISO 5000, it didn't blur and become pixelated. You can see the back ground more clearly.

ISO 50 would be best for when there is a lot of light in the place you taking the picture.
ISO 5000 would be best if there wasn't ENOUGH light, same with Flash but I wouldn't suggest flash for any smaller lights, like Christmas tree lights.