Phil Schiller, the SVP of Marketing at Apple, has revealed while responding to a tweet that the iPhone 7 Plus will not always use its secondary telephoto lens when using the 2x optical zoom.
The executive says that in low light, the iPhone 7 Plus will make use of its primary 12MP f/1.8 aperture OIS lens to take the best photo. The secondary telephoto lens features an f/2.8 aperture and lacks OIS, which does not particularly make it suitable for shooting pictures in low-light. Thus, using the primary 12MP sensor with its wider f/1.8 aperture in such scenarios makes sense as it will be able to take better photos. In such cases, if the iPhone 7 Plus uses its secondary telephoto lens, the photos will come out dark and with very little details that would make them of little use.
No Issue. In low light (or 10cm macro) the dual camera may use the wide angle 28mm f/1.8 + OIS lens for the best photo #iPhone7Plus
— Philip Schiller (@pschiller) September 17, 2016
The telephoto lens has a focal length of 56mm which means that it cannot be used to focus on subjects that are within 10cm of it.
Since in low-light, the iPhone 7 Plus will use the primary lens to take photos, it also means that zooming in beyond 2x will lead to the phone producing photos that are more grainy and noisy than usual since the secondary telephoto lens is not used.