Dan Provost of Studio Neat (makers of the excellent Glif) ran some tests to analyze the low-light performance of the iPhone X’s telephoto lens:

Last year, when the iPhone 7 Plus was released, Glenn Fleishman wrote a terrific piece for Macworld about how the dual lens camera system works. In short, when you zoom-in to 2X, the camera does not always switch to the telephoto lens. In some cases (typically in low light scenarios), you will be presented with a cropped image from the wide angle lens instead. This was sacrilege to camera nerds, but Apple would argue that if the cropped image looks better in those low light situations, then that is the correct approach.

Results are impressive:

As you can see, the iPhone X required very little light before it decided to use the telephoto lens. The iPhone 7 Plus required quite a bit more. I used the app Light Meter to measure the light at each interval, which I denote in the video. The app measures the lux, which is a measure of illuminance equal to one lumen per square meter. (I measured from both devices and averaged the results, as the readings were slightly different. I wouldn’t expect an app to function as well as a true light meter, but this probably gets us in the ball park).

Make sure to check out the video to see the lens switching in action. The difference between the iPhone 7 Plus and the X is substantial when it comes to the amount of light required for the system to pick the telephoto lens.