The algorithms of the autofocus USB camera module mainly include contrast detection autofocus (CDAF), phase detection autofocus (PDAF), and hybrid autofocus algorithms. The following is a detailed introduction:
Contrast Detection Auto Focus (CDAF)
Principle: The focus position is determined by calculating the contrast between adjacent pixels in the imaging. Specifically, the algorithm acquires the statistical information FV (Focus Value) from the Image Signal Processor (ISP) and calculates the distance and direction that the lens will move next time.
The process: CDAF is an iterative and gradually converging process, similar to the manual focusing process, that is, blur – clear – blur, and then gradually balance to the clearest position. During this process, the system first calculates the contrast degree, and then moves the camera and compares the contrast degrees based on the calculation results until the position with the maximum contrast degree is found.
Features:
Wide applicability: It has good adaptability to various types of lenses and imaging conditions, and is one of the most popular and widely used autofocus technologies at present.
Relatively low cost: Due to its relatively simple and easy-to-implement principle, CDAF has a cost advantage, which has led to its wide application in entry-level digital cameras and smart phones and other devices.
Light and contrast sensitivity: It has a relatively high sensitivity to light and contrast. In low-light conditions, due to the reduced image contrast, the focusing speed may be affected and become slower.
Phase Detection Autofocus (PDAF)
Principle: Some special masking pixels are reserved on the photosensitive element of the imaging sensor, and these pixels are specially designed for phase detection. The design of masking pixels is similar to human eyes. They respectively cover the left and right halves of the pixels, thereby simulating the Angle difference of objects seen by both eyes. When light is refracted through the lens, interference patterns are formed on the imaging sensor. The phase difference (PD value) of these interference patterns can be used to calculate the position of the focus.
Process: By comparing the light intensity differences in each pair of pixels, the system can calculate the phase difference. If the focus position is inaccurate, the system will adjust the position of the lens according to the size and direction of the phase difference to move it in the correct direction. The calculation of phase difference and the determination of focusing position are usually based on some mathematical models and algorithms, such as cross-correlation methods or sharpness detection methods. Once the system determines the inaccuracy of the focusing position and its adjustment direction, it will control the camera lens to make fine adjustments until the lens moves to the correct focusing position.
Features: It enhances the speed and accuracy of focusing, enabling digital cameras, video cameras and other devices to better capture clear and sharp images.
Hybrid autofocus algorithm
Principle: It combines the advantages of contrast detection and phase detection to enhance the focusing speed and accuracy. For instance, in some cases, phase detection can be used first to quickly locate the approximate focusing position, and then contrast detection can be employed for fine adjustment.
Feature: The hybrid autofocus algorithm can fully leverage the advantages of both focusing methods, providing faster and more accurate focusing performance.