iLounge:

The dual lens system on the iPhone 7 Plus is a pretty useful feature — you get the benefits of an 2X optical zoom whenever iOS decides it’s appropriate — but unfortunately due to the differences between the two lenses in both aperture and physical position, this can cause a “glitch” in your video if your iPhone switches to or from the second lens as you’re zooming in or out. The good news is that Apple has tucked away a setting that will allow you to avoid this problem by preventing the iPhone from automatically switching lenses during video recording.

This is a great tip for those of you who shoot video with your iPhone 7 Plus.