There are traditionally two methods for achieving zoom in cameras: digital zoom, which programmatically crops the desired area, and optical zoom, which uses lenses. In classic cameras, optical zoom involves mechanically moving lenses within the lens. However, in smartphones, optical zoom is often implemented differently. A newer option is hybrid zoom, which combines both digital and optical zoom.

Digital zoom

Digital zoom is a software option for enlarging an image, which involves cropping a specific area of a picture from the entire field of the image. To put it simply, the smartphone takes a photo conditionally on the main rear camera, crops out the required piece of it and scales it to the original frame size. The area around the selected part is cropped off as unnecessary, and all manipulations with the photo are carried out by software methods.

When using digital zoom, a certain area of the entire image is cropped out and stretched to the original image size.

Digital zoom, being a simple crop, leads to a loss of detail and sharpness in photos, especially at high software zoom ratios, resulting in blurry and unclear images. However, a solution to this issue is found in the ongoing "megapixel race".

In modern smartphone cameras, there is a tendency to significantly increase the number of megapixels - the number of points on the matrix, which are directly responsible for image formation. On the one hand, multi-megapixel cameras provide increased image detail when shooting in low ambient light conditions. In practice, this is realized thanks to the binning technology — combining neighboring pixels in groups of 4, 9, 16 or more points to increase light sensitivity. In other words, a 64 MP camera shooting in 4-pixel binning mode will produce 16 MP images. This is discussed in more detail in the material “Megapixels in smartphone camera: is their number so important?”.

On the flip side, high-resolution cameras enable relatively seamless cropping of images when using digital zoom. For instance, a 12 MP resolution results in a photo with dimensions of 4000x3000 pixels — suitable for post-processing and printing. If the main camera boasts a 64 MP resolution, the larger image size of 9000x7000 pixels allows for easy extraction of a 4000x3000 pixel segment without a significant loss in quality.

When cropping images from high-resolution cameras, you can achieve acceptable digital zoom quality. But only if the image size is reduced.

As for the digital zoom ratio, it varies widely. In marketing material, you can often see figures like " 100x digital zoom", but in practice, it is a digital zoom that has limited usage. Each step of the digital zoom reduces the image quality. Therefore, it makes sense to use it if you need to shoot photos with a small zoom (up to 5x). At high zoom ratios, the image will simply fall apart into pixels.

Smartphones with good camera zoom

Optical zoom

Optical zoom differs from digital zoom in its approach to creating images. When using it, zooming is done before the photo is taken by changing the actual focal length. A group of lenses inside the lens takes part in this, by moving which the scaling effect is achieved. The lenses reduce the angle of view and thus bring the subject closer, the camera sees the picture already scaled and takes the photo without loss of quality.

In smartphones, what is commonly labeled as optical zoom is more accurately described as a sequential switch between the main camera sensor and a telephoto or periscope camera. This process involves setting intermediate zoom positions through software methods. Genuine optical zoom entails a smooth adjustment of the focal length within a specific range, such as 85-125 mm in the telephoto lens of the Sony Xperia 1 V smartphone.

It is difficult to fit a movable lens group into a compact smartphone body. It is not easy to make room in the case even for a good telephoto lens, the optical system of which has a more elongated shape. However, there is nothing impossible in it — mobile phone manufacturers have managed to invent a so-called periscope camera placed perpendicularly to the body (see the picture below). Light comes to its matrix by refracting the rays through a special mirror, thus using the entire sensor area to achieve the effect of optical zooming.

In order to accommodate a complex optical system within the smartphone's body, manufacturers have managed to invent a so-called periscope camera placed perpendicularly to the body.

Cameras with optical zoom are not cheap, and they are found only in some flagship smartphones. In addition, such photomodules have a complex design. The zoom ratio of periscope cameras is usually low (up to 3x, rarely 5x). However, they are still better than digital zoom.

Smartphones with optical camera zoom

Hybrid zoom

And now we come to the most interesting part. Multi-cameras in modern smartphones have made it possible to implement the hybrid zoom in practice. Let's say a mobile phone has a main camera sensor and a telephoto camera with an optical zoom of 2x. Shooting will be done in the following manner:

  • in the zoom range 1x – 1.9x, the smartphone crops the image from the main sensor;
  • at the 2x mark the phone switches to telephoto shooting;
  • zooming in over 2x is carried out on a telephoto using digital zoom.

*Instead of the 2x multiplier, it's possible to substitute any other optical zoom ratio prescribed in the specifications of a particular smartphone.

Hybrid zooming involves a combination of optical zoom and digital zoom methods.

A simple combination of different camera modules and digital zooming provides a better result. After all, for a conditional zoom ratio of 4x it would be necessary to crop out a small part from the entire area of the image of the camera's main sensor. And when using a telephoto zoom, it is supposed to crop a halved area — the primary optical zoom of 2x is already obtained.

Hybrid zooming is implemented in another way. And artificial intelligence algorithms are directly involved in this. Take, for example, the option when a smartphone uses various sensors when shooting to simultaneously capture details from several cameras. The information obtained from them can be used to intelligently improve photographs with digital zoom. Such pictures do not reach the level of a true optical zoom of the camera, but small details are drawn much better in them than when using basic digital zoom.

Hybrid zoom often uses data from multiple image sensors at once, combining them into the final shot.

In the process of creating images with hybrid zoom, interpolation methods are frequently employed to artificially enhance image resolution by inserting extra pixels. These additional points are interpolated between original pixels, extracting color information from neighboring cells. While these methods make it easier to enlarge an image, the improvement in image quality is marginal, especially for zoom ratios below 5x.

Finally, hybrid zooming also applies to optical zoom models. A camera with optical zoom of conditional level 5x performs the primary zooming of the image, and the software adds another zooming, allowing to achieve an impressive zoom ratio up to 100x.

Smartphones with optical camera stabilization

Smartphones with variable optical zoom ratio

Of the wide range of smartphones, only a few mobile devices can boast a camera with variable optical zoom ratio. In all other cases, we are talking about a simple switching between rear cameras to achieve the effect of optically enlarging the image. The intermediate approximation is achieved by software methods.

For convenience, we have compiled a current list of smartphones with variable optical zoom ratio:

A telephoto lens with a variable set of equivalent focal lengths of 85-125 mm is the signature feature of Sony’s flagships from the Xperia 1 line.

...And essentially that's all. From time to time, a variety of smartphone concepts with real optical zooming appear, but they are still not widespread in hardware.



There is definitely a future for optical zoom technology in smartphones. For now, optical zoom is often referred to as shooting with a telephoto lens or its more advanced type — a periscope camera. True, it achieves a constant zoom ratio (2x, 3x, 5x or even 10x), but all intermediate zoom positions are finished by software methods, for which artificial intelligence algorithms are often used.