Most modern high-end mobile phones now have multiple cameras. The front and back cameras are easy to understand given the popularity of traditional photos and selfies, but it can be difficult to see why adding more cameras to a phone is useful.
When a photographer wants to change the zoom level for a photo, they can generally adjust the lens to some degree. The careful change in the positioning of the lenses allows the camera to zoom in or out with no loss of image quality. With mobile phones though, this isn’t really possible. Given that phones are pretty thin, there just isn’t space to fit complex optics that allow for a decent range of optical zoom options.
Digital zoom can be used rather than optical zoom, however, this always results in reduced image quality that is very noticeable in anything other than very small changes.
Tip: Optical zoom refers to the changes in the position of the actual lenses used to focus the image on the camera sensor. Digital zoom refers to a post processing technique that tries to “zoom and enhance” the image, but this just isn’t possible in the way that Hollywood portrays it and always results in reduced image quality.
To be able to offer more optical zoom levels, phone manufacturers include a second camera with a different lens designed for a different level of optical zoom. With these two cameras, the phone can switch between them allowing a larger range of optical zoom options. This is very similar to a photographer swapping the lens on the camera or switching to a camera with a different lens equipped.
Tip: Even with the limited size, there are some minor adjustments that can be performed to the lens allowing a small range of zooms. With multiple cameras, these zoom ranges are designed to intersect to allow a smooth handover.
A wide-angle lens has also seen more use recently allowing phones to zoom out further to capture wider scenes.
Folded optics is a design technique that can be used to partially overcome the size restrictions on modern phones. Traditional optics requires all lenses to be placed in a straight line, where the lack of thickness of the phone and the presence of its other components quickly prevent further expansion limiting optical zoom options.
Folded optics, on the other hand, use a mirror or prism to redirect the light vertically within the phone chassis, allowing the lenses to be placed in two perpendicular lines. There is much more vertical space within a phone that can be used for camera optics. This allows for a much greater range of optical zoom configurations to be used while not taking up too much extra space.
Other uses of multiple cameras
By combining the data from two separate cameras placed close together, as they would be on a phone, it is possible to estimate the distance to objects in the scene. By using this depth data, it is possible to apply a blurring filter on the background, as would normally be seen in a photo taken with a short focal length. This is generally advertised and used as a way to enhance photos of people and selfies.
The ability to use multiple cameras to estimate the distance to items is also useful for augmented reality applications. Here the depth readings can be used to help place virtual items in the real world and maintain their location.