The iPhone XS already records some pretty stellar low-light video, but if you want even better results, there’s a setting that you can tick that will give you better exposure in dimmer situations.
Daring Fireball’s John Gruber points this out in his iPhone XS review:
“There’s a cool XS-exclusive feature in Settings → Camera → Record Video. If you’re shooting at 30 FPS — whether in 720p, 1080p, or 4K — you can enable “Auto Low Light FPS”, which will drop the frame rate to 24 FPS on the fly whenever the phone deems necessary to get better low light exposures. This can happen in the middle of recording. Start recording in a bright room and move to a dark one or turn down the lights, and the frame rate will change within the clip.”
It’s worth noting that you won’t see the setting if you’ve got your camera set to record at 60 FPS, so you’ll first want to enable “1080p HD at 30 fps” or “4K at 60 fps” under Settings -> Camera -> Record Video before you’ll see the toggle for “Auto Low Light FPS”.
Some hawk-eyed video enthusiasts may be able to notice the change in the frame rate, but most people probably won’t notice the difference. What they will notice, however, is the better low-light exposure in a video during times when it’s really needed.