Why our smartphones don't really "zoom" when we take a picture

Zoom 2x, 3X, 50x, 100x… Smartphone manufacturers do not hesitate to boast about the photo capabilities of their various gadgets. But the use of the word "zoom" is rightly debated.

Seeing a smartphone with 2 or 3 different photo modules has almost become commonplace today. In recent years, manufacturers have multiplied the lenses on the back of smartphones to offer more versatility in the photographic exercise.

The iPhone 13 Pro, for example, has three large cameras on the back. An ultra wide-angle sensor, a wide-angle sensor (which is the default) and a telephoto lens capable of offering a "3x optical zoom forward" according to Apple. Yet the iPhone, like most other phones, doesn't really “zoom”.

What is a zoom?

Going by Wikipedia's crude definition, a zoom is "a varifocal lens" that "moves one or more lens groups inside the lens, which continuously changes the magnification, therefore the field angle covered by the lens. To understand why we misuse the word "zoom" on our smartphones, it is necessary to decipher this definition.

On a digital camera or a video camera, we traditionally zoom using a zoom ring, a physical mechanism that will move the different lenses within the lens. In doing so, the light will be concentrated differently through the glass elements, offering the possibility of seeing a subject more closely, as a telescope does for example.

This is called an optical zoom, since it involves the manipulation of certain optical properties. The angle of view corresponds to the "frame" of his photo. GoPro cameras, for example, have a very wide angle of view, while the telephoto lens of the iPhone has a much tighter angle of view.

In the “traditional” definition of zoom, therefore, a single lens and a single photo sensor are used, only the lenses physically move to adjust the frame. It is on this crucial point that cameras and smartphones differ.

Why don't we "zoom" on a smartphone?

Most phones on the market are equipped with what is called a fixed focal length. That is to say, unlike a camera zoom, the lenses do not move within the cameras of our mobiles. There is no physical mechanism that will alter the alignment of the parts. As a result, a smartphone camera only has one angle of view.

To allow our smartphones to adopt several angles of view, manufacturers have simply added lenses with different lens alignments. Each camera has a fixed focal length, but offers different degrees of magnification. There is no physical movement, like on a traditional zoom: the phone will just move from one photo module to another. Simply put, it's like changing lenses on a digital camera, but automatically.

The focal length (expressed in mm) expresses the field covered by a lens. The smaller it is, the wider the field will be, conversely

To take our example of the iPhone 13 Pro, we therefore have an ultra-wide-angle lens (with a focal length of 13 mm), a wide-angle lens (26 mm) and a telephoto lens (77 mm). The phone will jump between these different modules to offer different settings. But there is no optical zoom to speak of, since the parts do not move.

What does it change ?

The problem with this pseudo-zoom technique is that each photo module will use its own parts, the quality of which may vary.

On a camera zoom lens, regardless of the angle of view chosen, the material does not change. It's always the same sensor (where the light is “printed”) and the same lenses that are used, but in different configurations. On a smartphone, each module has its own lenses and its own sensor.

As a result, the sensor of a telephoto module can be a little less than that of the "standard" photo sensor. This is also often the case, because these modules are less used than the default one. The quality of the lenses can also vary.

A question of vocabulary

Technically, then, smartphones don't zoom like a camera can. They just change photo modules. But can we really say that it is deception? Not really, since the result is the same, or almost: you get closer to your subject with a different lens alignment.

After all, we also talk about “digital zoom” when we crop a photo. Yet there is no lens movement. The word zoom is used to cover a wide range of different reality today. Purists might choke on hearing about optical zoom on an iPhone, but the reality is that while the techniques are different, the result is the same or nearly the same. Maybe tomorrow our smartphones will really zoom.