All-pixel Autofocus?
Autofocus on smartphone cameras in general takes phase difference detection.* This means that some pixels on the sensor are dedicated to the detection of phase differences. With this method, the image signal must compensate the areas corresponding to the detection pixels. However, too many detection pixels make it difficult to compensate the gaps and compromises the image quality. Therefore, there is a limit to the number of pixels assignable to phase difference detection, which in turn limits the AF performance.
The “all-pixel AF” addresses this challenge by enabling all pixels on the sensor to perform both image sensing and phase difference detection. When all pixels are used as image sensing pixels, image signals need no compensation. The phase difference detection mode is designed to have no impact on image quality.
* A method to detect an optimal focus point based on the information from several incident light beams of different angles.
Advantages of All-pixel AF
With the significant improvement of phase difference detection, AF has been enhanced to provide stable performance even with the scenes that were difficult to find a focus previously. The sensor can focus on the target anywhere within the view frame. Moreover, the enriched phase difference information means reliable AF performance even in a low-illuminance environment.1.Reliable AF performance even in a low-illuminance environment.
Technology behind the All-pixel AF
Sony has developed several method, to realize the all-pixel AF to be offered.
| (1) | Dual PD method | : | the basic type sensor that offers all-pixel AF |
| (2) | 2 x 2 OCL method | : | adopted for medium sized sensors with miniaturized, high-resolution pixels |
| (3) | Octa PD method | : | adopted for large sized sensors to achieve high sensitivity and high resolution |
For smartphones, the best method, is adopted depending on the purposes of use.
(1)Dual PD method
The dual PD (photo diode) takes two adjacent photo diodes to form one pixel. They share one on-chip lens (OCL*), so they catch different incident lights, which enables to detect phase differences. The signals from both PDs are combined to be used for image data, which cancels the impact of phase difference detection. The Dual PD method is used as a default system for all-pixel AF.
* Condenser lens to collect light
Bayer
One PD under one OCL
Dual PD
Two PDs under one OCL
(2)2 x 2 OCL method
The 2 x 2 OCL method is based on the Quad Bayer sensor, in which four pixels of the same color use one OCL. Like the Octa PD method, this method, benefits from both the Quad Bayer functionality and all-pixel AF. Unlike the Dual PD method, it does not require the signals from two PDs to be combined for image signals, which makes it easier to achieve higher resolutions. The method, is therefore more suitable for medium sized sensors.
Quad Bayer
2 x 2 OCL
(3)Octa PD method
The Octa PD method is a hybrid between Quad Bayer sensor and Dual PD method. It retains the features of the Quad Bayer arrangement, such as high sensitivity and HDR compatibility, while achieving the excellent AF performance through the all-pixel AF functionality, making it more suitable for large sized sensors. Its unique feature is in the HDR mode, that phase differences are detected in all pixels across the three exposure levels (long, medium, and short), enabling quick autofocus even on a low-luminance target.
Quad Bayer
Octa PD
*SDR stand for Standard Dynamic Range.
Source: https://www.sony-semicon.co.jp/e/products/IS/mobile/autofocus.html
