With the introduction
of iPhone 7 Plus with Dual Camera Lens system, it lets you snap beautiful
photos with blur effect (depth-of-field) like DLSR.  
iPhone 7 Plus dual camera lens system is amazing
as it gives the benefits of 2X optical zoom with 12MP whenever iOS decides it’s
appropriate but unfortunately due to the differences between the two
lenses in both aperture and physical position, which also affects image quality
and can cause a “glitch” in your video if your iPhone switches to or from the
second lens as you’re zooming in or out.

If you begin
recording a video while zoomed in to beyond 2X, you may not be able to zoom
back out, as you’ll already be using the 2X lens. Keep in mind, however, that
there are a number of factors that iOS uses to determine whether it should use
the optical 2X lens or simply rely on digital zoom, such as in lower light
conditions where the faster primary lens has an advantage.

Apple has added a Lock Camera Lens feature in the setting which avoids your iPhone 7 Plus from automatically switching lenses during video recording

To prevent
this issues, Apple has added a Lock Camera Lens feature in the setting which
avoids your iPhone 7 Plus from automatically switching lenses during video
recording.

Also See:

Here’s how to
disable the automatic lens switching on iPhone 7 Plus while recording videos.
1) Launch
the Settings app.
Apple has added a Lock Camera Lens feature in the setting which avoids your iPhone 7 Plus from automatically switching lenses during video recording
2) Tap Photos
& Camera
.
3) Under Camera Section,
Tap Record Video.
Apple has added a Lock Camera Lens feature in the setting which avoids your iPhone 7 Plus from automatically switching lenses during video recording
4) Now
towards down, toggle ON the switch Lock Camera Lens.
That’s it.

Enabling Lock
Camera Lens switch disables your iPhone 7 Plus automatically switch between
camera lens while recording video. You can now record video on iPhone 7 Plus with
actual images, stabilization without switching lens automatically with best
results.