IPhone 7 and Zen Zoom, how optical zooms work

Thanks to the dual rear camera, even smartphones can take photos with optical zoom and not just digital

Until not so long ago this was an exclusive feature of cameras, regardless of whether it was an SLR or a small compact. Thanks to their set of lenses and lenses, which allow you to "move closer or further away" the subjects of the photo without affecting the quality of the images.

Nowadays, however, the optical zoom is starting to peep into the world of smartphones. Several manufacturers, in fact, have equipped their flagship models of late 2016 and early 2017 with this new feature mutated directly from the world of photography. These are, at the moment, limited optical zooms (in the iPhone 7 Plus, among the very first to implement it, the magnification factor is "just" 2X) but still functional for the purpose. The user, in fact, will be able to photograph in detail even subjects at medium distances without a loss of image quality.

Difference between optical zoom and digital zoom

The biggest difference between optical zoom and digital zoom lies in the quality of the photos you take. In the first case, in fact, the enlargement is the result of the movement of the lens lenses that "modify" the incoming light before it touches the photographic sensor (or the film, if you are using an analog camera). This allows you to take photos with a magnification factor in double digits (depending on the telephoto lens used you can get 10x or 15x magnification) without the image quality being affected in the slightest.

In the case of digital zoom, the enlargement of the subject is obtained via software: the camera (or smartphone) crops the unenlarged image until you get the result you want. All this, however, to the detriment of the quality of the photo: as the enlargement increases, in fact, the "cropping" factor of the original subject will also increase. In order to avoid obtaining photos that are too small, the algorithms will "fill" the images with pixels similar to those that already exist, while grazing the image. In short, be careful how you use the digital zoom: the end result may not be very appreciable.

How the optical zoom of smartphones works

The optical zoom of smartphones such as the aforementioned iPhone 7 Plus, Asus Zenfone Zen Zoom or the OnePlus 5 is the result, instead, of the dual rear camera. The two sensors, in fact, have a different focal length and allow the photo app to enlarge the subject of the photo without affecting the quality. The operation of the optical zoom, therefore, is quite simple: the smartphone will use one or the other sensor depending on the magnification factor required by the user. Unlike cameras, however, the optical zoom of smartphones is "fixed": you can have a magnification factor of 2x or 5x, but not intermediate values. If you want to magnify the subject by 1.6x, for example, you'll have to rely on software that manages and "merges" the images taken simultaneously by the two sensors.

The optical zoom, however, is not the only mode of magnification allowed by the dual rear camera. In case the two sensors have a different resolution (20 megapixels and 12 megapixels, just to make an example) you can take advantage of the so-called hybrid zoom: using special image processing algorithms, the phone is able to produce a zoomed image without loss of quality. Just "merge" the photos taken at the same time by the two lenses and you're done.