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The S23 Ultra features a new Quad Pixel Phase Detection Auto Focus (PDAF) technology. This has been made possible by the S23 Ultra's change from a Dual Pixel (2PD) sensor to a Quad Pixel (4PD) sensor. The Dual Pixel sensor could only detect phase difference horizontally. The revolutionary new Quad Pixel sensor can detect phase differences in both horizontal and vertical directions.
S23 Ultra's auto focus algorithm has evolved to a new level. Thanks to the sensor change, namely the Quad Pixel PDAF that operates on phase difference information from multiple directions.
First, let's look into the background of this technology.
Problems with Dual Pixel (2PD) Sensor in Phase Detection AF
As previously described, in the phase detection AF method, the Dual Pixel sensor has the following structure, which allows it to detect phase differences for horizontal elements.
If you are likely to shoot a subject in a typical setup, the horizontal phase difference is enough to put the AF into motion. In other words, for most subjects, the horizontal phase difference is enough to trigger AF. However, in a few specific situations, there are subjects whose horizontal phase difference components are not detectable. They might look like the following.
In the photo above, the user wants to focus on the tree branch lying horizontally in the centre of the photo. However, the phase difference information on the horizontal branch element is missing, which takes the focus away from the branch to the buildings behind, completely defeating the intention behind the shot.
Introducing a sensor to detect phase difference components in the vertical direction
The Quad Pixel (4PD) sensor was introduced so as to detect phase difference components in the vertical direction.
As shown in the figure above, you can see that a single sensor generates two pieces of phase difference information. However, a simplistic approach to this phase difference information can lead to unintended action/behavior if the left/right and up/down phase difference information is significantly different.
In the photo above, the Quad Pixel sensor works but it is difficult to distinguish and select between the two phase difference information. The left/right phase contrast information targets the writing at the back, while the up/down phase contrast information targets the holding bar at the front. Situations like these may trigger unwanted AF action.
Quad Pixel Auto Focus Algorithm based on Two Phase Difference components
To prevent the above situation, the AF algorithm evolved accordingly with the introduction of the Quad Pixel Sensor. By combining data from AI AF, scene segmentation, and laser, we have selected and aggregated optimal phase difference information that fits user requirements and enables the camera to focus accordingly. This solves the problems of the existing Dual-Pixel-based Auto Focus and provides users with a more advanced AF usability.
You can check out the functionality of the Quad Pixel PDAF Algorithm applied to S23 ultra in the picture above. The camera can focus accurately by picking out the differences between the existing Dual Pixel AF information on just the horizontal components (the ballpoint pen photo) and the newly applied vertical and horizontal phase difference information on the main subject.
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