Dan Provost from Studio Neat has gotten nerdy with iPhone X’s camera system:
I created a test to hopefully get a rough idea of how much light is required before an iPhone 7 Plus and iPhone X decide to switch to their respective telephoto lenses in 2X mode. I placed an object (in this case, an old Rolleiflex camera) on a white backdrop, and flanked it on both sides with two LED studio lights. I set up the iPhone 7 Plus and iPhone X on tripods (using the Glif, natch) and positioned them to keep the framing as similar as possible. Then, starting from a completely dark room, I slowly raised the light levels and observed when the lens switched on each camera.
I love this.