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.

With the improved sensor, wider aperture, and added optical image stabilization on the telephoto lens in the iPhone X, I wanted to see if Apple’s frequency of presenting a cropped image at 2X was reduced at all. The answer: yes. But by how much?

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.

The results are in the video below.


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).

I also used the manual camera app Halide to get an ISO and shutter speed reading at the various light levels, and came to the conclusion that the iPhone X requires roughly 2 fewer stops of light before switching to the telephoto lens, as compared to the iPhone 7 Plus. This is obviously great news, and speaks to how improved the second lens is after just one year. In my own use of the phone for the past couple weeks, it does indeed seem to be the case that I am very rarely presented with a 2X cropped image. As Glenn points out in the aforementioned piece, the easiest way to test which lens is in use is simply by covering it with your finger.

And while we’re on the topic of the dual lens camera system, one stray observation. Previously, while shooting video, you could jump freely between lenses during a recording (it would do a very quick zoom and switch to the other lens at the tail end of the animation). This is still the case, except when recording 4K at 60 FPS. In that mode, you must select which lens you want to use before recording, and then you can only digitally zoom while recording. I think there is a pretty clear explanation for this: in order to switch freely between lenses while recording, both sensors need to be “on,” and 4K 60 FPS is simply too much for the phone to handle two streams at once. Perhaps in a year or two this will become an option again.