A closer look at Xiaomi's Mi 5 camera

To get things started, we look at the specifications of the camera. Xiaomi Mi 5 uses the Sony IMX298 sensor to capture photographs. It is a diagonal 6.521mm [Type 1/2.8 Sensor] 4608 x3456 16 Mega-pixel CMOS active pixel type stacked image sensor. It also adopts Sony Exmor RS technology to achieve high speed image capturing by column parallel analog-digital converter circuits processing to produce high sensitivity and low noise image.

What is Sony Exmor RS?
The Exmor RS is a Sony's improved CMOS image sensor achieves high-resolution, high-performance and compact size by having a stacked sensor technology. With a higher density of sensors that processes analog to digital conversion, a overall more accurate image can be produced.

In addition the lens of the camera unit also contributes to its success. However, there are no information on Xiaomi Mi 5's lens make. Usually, competitors like Sony, will release full information about their lens type. For example, Sony Z and latest X series uses their own in-house G-Lens and Sensor.

An important feature of the Mi 5 camera is its 4-axis optical image stabilization (OIS). It aims to reduce camera shakes, thus improving night shot quality with a longer exposed  shutter speed. The video below shows the optical image stabilization (OIS) technology as compare the iPhone 6S and iPhone 6S Plus.

Sony has introduced the new spatially multiplexed exposure technology, allowing more achievable high dynamic range still pictures and movie. This results in Xiaomi Mi 5's capabilities to to capture images with rich detail and sharp focus.

The IMX298 image system block diagram is shown below:

Other than Sony's sensor technology, Qualcomm has also integrated a new Visual Processing Technology to be on the 820 SoC. The next-generation Spectra ISP is said to bring the most Advanced Visual Processing Technology to Snapdragon 820 Processors.

The Snapdragon 820 will debut with the new 14-bit Qualcomm Spectra™ image signal processing (ISP) unit, designed to support superior DSLR-quality photography and enhanced computer vision.

The state of the art dual ISP design produces superior camera image quality and experiences. Qualcomm Spectra supports the latest 14-bit sensors for a wider range of colors and more natural skin tones. Users can also expect better photos through the hybrid autofocus framework and multi-sensor fusion algorithms that support next generation computational photography.

Qualcomm Spectra ISP is Qualcomm Technologies’ most advanced dual-imaging signal processing unit to-date, integrated and designed to provide best-in-class camera image quality and end-user benefits, including:
  • Superior image quality, with more natural skin tones via advanced, 14-bit dual ISPs supporting up to 3 simultaneous cameras (e.g. one facing the user, and two rear facing), and up to 25 megapixels at 30 frames per-second with zero shutter lag;
  • Improved photos with Qualcomm Spectra ISP’s flexible hybrid autofocus framework and multi-sensor fusion algorithms supporting next generation computational photography;
  • Improved power efficiency when compared to previous generations, better noise immunity and higher throughput via advanced compression techniques and use of the latest MIPI serial C-PHY interface; and
  • Next generation Computer Vision and other use cases via direct-to-DSP raw bayer data streaming and pre-processing capabilities.

Sony Sensor Technologies

Sony seems to reserve their best image sensors for their own branded smartphone.

Over the 4 generations of smratphones from the Sony Z1 to Z3+ Sony has equipped them with the IMX220 image sensors. The new Sony Xperia Z5 and Xperia X is upgraded to the new Sony IMX300 image sensor which was released on September 2015.

The Sony Xperia Z5 and Xperia X Performance features a 7.87mm (1/2.3") sensor, which is typically bigger than other smartphones. However, they do not have any OIS technology included for their rear camera. 

Typically, if the angle of view is at 24mm and below, OIS is not that crucial as camera shake will not affect image quality too much. OIS is very important for angle of view above 50mm. Perhaps, Sony considers 24mm to be wide and do not require OIS capabilities. 

The wider the field of view, the less important OIS is for the photo shot. However, it could be better to have OIS technology in scenarios where there is a need to capture photos with slow shutter speed and when in a dark location. 

There are reasons for manufacturer to skip on OIS. The inclusion of the technology will increase the overall cost of the camera. The size of the lens assembly will also be increase. This may affect the quality of the photo (e.g sharpness). Wide angle lens do not come with OIS technology, as these lens are not used for video shooting, which effects of OIS are most prominent.

Why is a bigger sensor is better?
With a larger image sensor, a larger surface area is used to capture light. This allows a higher density of data capture, thus allowing a clearer image. A larger surface area also means increased light sensitivity and better noise control at higher ISO.  The inherent noise is therefore relatively lower compared when at higher voltage, thus increasing the signal-to-noise ratio (S/N).

Let us see which smartphone has the biggest sensor.

Sensor Size
Sony IMX220
Sony Xperia Z1 –Z4
5264 x 3960
Sony IMX298
Xiaomi Mi5
4608 x 3456
Sony IMX286
Huawei P9/ P9 PLUS
3968 x 2976
Sony IMX300
Sony Z5 & X Series
5984 x 4140
Sony IMX377
Google Nexus 6P
4032 x 3024
Sony IMX260
Samsung S7
4032 x 3024
Sony IMX - Unknown
Apple iPhone 6
3264 x 2448
Sony IMX - Unknown
Apple iPhone 6s
4224 x 3024

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