What is the principle of digital camera autofocus?

Auto-focusing refers to the way that the reflected light is received by the sensor CCD on the camera and focused by the computer by using the principle of object light reflection. It can be divided into two categories: one is active and the other is passive. From the basic principle, autofocus can be divided into two categories: one is ranging autofocus based on the measurement of the distance between the lens and the object being photographed, and the other is focusing detection autofocus based on clear imaging on the focusing screen. Ranging autofocus: Ranging autofocus mainly includes infrared ranging and ultrasonic ranging. The principle of infrared ranging method is that the camera actively emits infrared light as the ranging light source, forming the geometric relationship between infrared light-emitting diodes, and then calculating the focusing distance. The ultrasonic ranging method is based on the ultrasonic propagation time between the digital camera and the object. Digital cameras are equipped with ultrasonic transmitting and receiving devices respectively. When working, the ultrasonic vibration generator emits continuous ultrasonic waves. After the ultrasonic wave reaches the subject, it immediately returns to the receiver for sensing, and then the integrated circuit calculates and determines the focusing distance according to the round-trip time of the ultrasonic wave. Infrared autofocus and ultrasonic autofocus use active light or sound waves to measure distance, which is called active autofocus. Focus detection autofocus focus detection methods mainly include contrast method and phase contrast method. This method realizes autofocus by detecting the contour edge of the image. The clearer the contour edge of an image, the greater its brightness gradient, or the greater the contrast between the scene and the background at the edge. On the other hand, for out-of-focus images, the contour edge is blurred and the brightness gradient or contrast is reduced; The farther away from the focus, the lower the contrast. Using this principle, two photodetectors are placed at the same distance in front of and behind the CCD, and the image of the photographed object is imaged on the two detectors at the same time after being split, and the imaging contrast is output respectively. When the absolute value of the contrast difference between the two detectors is the smallest, it means that the focused image plane is just between the two detectors, that is, close to the imaging plane of CCD, thus focusing is completed. B-phase method is a method to realize autofocus by detecting the offset of the image.