The digital zoom takes advantage of the high resolutions available on the iPhone to provide an electronic zoom. This is accomplished by digitally cropping a source image.
To enable digital cropping a 4K image is used as a source. When zoomed out the entire 4K image is used to generate the 1080p output. When zooming in, the 4K image is digitally cropped to create the illusion of a mechanical zoom.
To use the digital zoom first make sure you are a 1080p recording format then Tap, Tap & Hold and Drag up and down on the viewfinder to zoom in and out.
Digital Zoom - Servo Control
The digital zoom servo control uses the distance between the start position (the location of the double tap) and the current end position to set the speed of the zoom. This type of control allows for a smooth zoom at a constant rate. Moving your finger further away from the start position increases the speed of the zoom, moving it closure slows it down.
Digital Zoom - Manual Control
The digital zoom manual control uses the distance between the start position (the location of the double tap) and the current end position to directly set the zoom. This type of control allows for fast direct manipulation of the zoom. Moving your finger away from the start position zooms the camera in, moving it back towards the start position zooms the camera back out. If you stop moving your finger, the zoom stops.
Digital Zoom - In & Out
Both the servo and manual control use the start position as a reference point for zooming. If you are at maximum zoom then moving your finger down the screen will zoom out. If you are at minimum zoom then moving your finger up the screen with zoom in.
The optical zoom takes advantage of the dual camera hardware in the iPhone 7 and iPhone 7 Plus. This hardware provides two separate camera modules that can be individually selected. One camera module provides a wide-angle lens and the other a telephoto lens (equivalent to 2x zoom).
If you use two fingers and double tap anywhere on the viewfinder you can swap between the wide-angle lens and telephoto lenses.
Unlike the digital zoom, which crops the image and is therefore limited to 1080p, the optical zoom uses two distinct lenses which both provide the same 4K (UHD) output. This means you can get super sharp close-up videos at 4K (UHD) resolution with no loss of quality.