The iPhone's telephoto limitations
Unfortunately, when it comes to shooting 2x photos, the telephoto's hardware weaknesses result in a handicapped lens when you're in poor lighting conditions — and that's where the iPhone's software takes over.
I was initially rather confused when, upon snapping pictures with the iPhone 7 Plus at a restaurant, the telephoto images didn't look all that much better than previous years' zoomed shots. When I looked at the metadata, however, things became more clear: The images I took were actually snapped with the wide-angle lens and digitally zoomed 2x.
At first, I thought this had to have been a mistake, or a goof or my part, but repeated tests confirmed my suspicions:
When the Camera app is in lower-light conditions (in my tests, anything under shutter speeds of 1/30-1/60 of a second), the telephoto will cede control to the wide-angle and its digital zoom. You can even test this for yourself by putting your finger over the wide-angle lens and entering 2x mode: If you're in good lighting conditions, you'll see your image, same as always; if you're in lower-light, all you'll see on the screen is a dark blob representation of your finger.
Source:
http://www.imore.com/iphone-7-plus-telephoto-wont-shoot-low-light-situations