The camera has become more and more important in smartphones and people have higher requirements for its focusing speed. A lot of methods to assist focusing are available in the current market, including laser, infrared, and phase focusing. Focusing is the process whereby a motor moves to bring the subject into focus. Focusing algorithms are divided into contrast detection and phase detection auto focus (PDAF). Let us look into detail on how PDAF Technology works.
In contrast focus (traditional auto focus systems), the moment you start your camera app, a lot of calculations are done by the processor to achieve the correct focus. First, the camera software triggers the lens to move and scan through its entire range of possible focus points, then it returns to whichever settings that provides the sharpest focus. Contrast detection requires the motor to move to different positions until the image displayed on the viewfinder shows the highest contrast. Contrast detection achieves precise focusing but takes a long time because the motor has to keep moving to find the highest contrast.
Phase detection is the process whereby the phase difference of the subject at the imaging plane is calculated to determine the distance between the motor’s current position and the position required for bringing the subject into focus. The motor then moves to this position.
Phase detection takes a short time, but the phase difference is easily affected by various factors such as noise. Therefore, phase detection is not suitable for focusing at low luminance, in a flat area, and on parallel patterns.
PDAF Technology uses paired masked pixels on the image sensor that are engineered to work just like your eyes; your left and right eye are separated by a distance, just like these paired pixels.
As shown above, the dark points represent the PD pixels. The phase information read using the PD pixels can be used to determine the corresponding focal length. After the GPU receives the focal length information, it can swiftly drive the lens to move to a position with the optimal focus.
PDAF Technology increases the focusing speed using planar image phase detection. This technology was first applied to the Interchangeable Lens Digital Camera, which allows the user to keep taking HD photos of fast-moving objects (children, pets, or cars) using autofocus. Compared to traditional focusing technology, the PDAF focusing speed has been greatly improved.
Traditional focusing technology had to detect image quality step by step until it achieves the clearest possible image with an optimal focus. PDAF focusing can detect the phase difference in an image and enable the motor to reach the correct position for the optimal focus in a single movement, eliminating the need to repeatedly check the image quality.
While PDAF technology is particularly effective in bright environments, for darker environments, Laser Auto Focus can save the day. As the name suggests, the smartphone has an infrared laser emitter and receiver built in. When you aim at an object, the smartphone blasts out a series of laser beam pulses. When the beam bounces off the object, the processor can then calculate the time of travel to the object and back, and then determine the distance. Laser Auto Focus is especially effective for shooting in low light since it’s usually too dark for the image sensor to see anything.
PDAF focusing technology, however, is not applicable to all scenarios because there is a lot of room for error. Therefore, hybrid focusing systems are commonly adopted with phase focusing used to find the correct position and contrast focusing used to achieve the optimal focus.