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Home > How To > What is the principle of autofocus?
What is the principle of autofocus?

What is the principle of autofocus?

By  Dido Toso 2020-01-18 844 23

The autofocus of the camera means that the lens automatically moves to adjust the focus according to the distance of the subject. The first auto-focusing camera was the Konica (Konica) C35AF camera produced in 1977. Two active focusing modes of infrared system and sonar system and two passive focusing modes of contrast system and phase detection system are introduced below.

Two passive focusing modes  

1. The infrared system belongs to the active auto-focusing mode, which emits infrared light through the infrared light-emitting diode, and the infrared light reflected after reaching the subject is received by the corresponding photosensitive element, and the distance between the subject and the camera is measured. it only takes 120 milliseconds to complete the focusing process. 

Infrared system was first used in the Canon AF35mm camera in 1979 and is now the most widely used (especially on the "fool" camera). 

Infrared system can focus not only in the dark, but also through the glass, as long as the optical axis of the glass and the lens is not 90. Right angle, target area as small as 1.5. (the target area of the contrast and sonar system is the vertical angle 11 and the horizontal angle 14.).

2. The sonar system is also an active autofocus. In 1978, Polaroid produced the first SX- 70AF primary imaging camera. When the shutter of the camera is pressed halfway through the electronic trigger, the sonar system can convert the four audio signals sent in sequence into ultrasonic waves, and then determine the distance between the subject and the camera according to the time when the ultrasound arrives at the target and then returns to the camera. The distance signal enters the receiver of the camera, and according to the accumulative pulse value of the register, the lens is set at the correct focus distance L. the whole process takes about 60 milliseconds and the focus range is 0.25mi 10m. The system can accurately measure and focus even at night, but it cannot be photographed through glass or water, and sometimes the near foreground will affect the accuracy of the focus.


3. The camera system is a passive focusing system. In 1977, the miniature photoelectric auto-focusing module produced by Honeywell (Honeywell) Company in the United States was first used in the Konica C35AF camera. The basic principle is similar to the double shadow coincidence rangefinder. the subject reflects the shadow to the two photoinductors in the machine through the reflector behind the two ranging windows (one fixed and one active). When the contrast of the two shadows is exactly the same, the output signal is the strongest, that is, focusing is completed, and the whole process takes about 80 milliseconds. This focusing method has the advantages of accurate focusing and low energy consumption. it is suitable for scenes with moderate brightness and contrast, and has a better effect in backlight. However, this kind of auto-focusing device also has some shortcomings: the accuracy is low in low light or in the face of low-contrast scenery, focusing errors can easily be caused in the face of objects of similar shape, and finger prints on the focus window will also affect the accuracy of focusing. The shooting subject must be in the middle of the picture, and the scene can not be changed at will after focusing, and the auto-focusing ability of the moving body is poor.

The principle of autofocus

Using the principle of light reflection of an object, the reflected light is accepted by the sensor CCD on the camera, and processed by computer, the way of driving the electric focusing device to focus is called autofocus. Most of them are divided into two categories:

1. Active: the infrared generator and ultrasonic generator on the camera emit infrared light or ultrasonic wave to the subject. The receiver on the camera focuses on the reflected infrared light or ultrasonic wave, and its optical principle is similar to the trigonometric ranging focusing method. There is also energy method in the initiative, which is used for autofocus of low-end popular cameras and widely used in all kinds of heads-up viewfinder cameras. Active focusing bevel, smooth and difficult to focus. It is difficult to focus on subjects with high brightness and long distance. This is because the light emitted is reflected in other directions or does not reach the subject. The active camera emits light or waves actively, so it can focus in low contrast and weak light. For thin lines of the subject, the moving body can automatically focus. The disadvantage is that it is difficult to focus when the subject can absorb light or waves, and it is also reflected by the glass, so it is difficult to focus through the glass.


2. Passive: the way of receiving and analyzing the reflection from the scene itself and focusing automatically. The advantage of this autofocus mode is that it does not need a launch system, so it consumes less energy and is conducive to miniaturization. The subject with a certain brightness can be auto-focused ideally, and it can also focus well in backlight. It can focus automatically on objects with high brightness in the distance. Can focus through the glass. But the disadvantage is that it is difficult to autofocus the subject with thin lines. It is difficult to focus in low contrast and low light. The auto-focusing ability of the moving body is poor. The auto-focusing ability of the subject with polarized light is poor. The focusing ability of black objects or mirrors is poor.


Active and passive autofocus have their own advantages, but there are two autofocus modes on SLR cameras, which can complement each other, switch automatically, give full play to their strengths and overcome their weaknesses. Passive focusing is often used on SLR cameras, so the focus is limited by the maximum number of apertures. It is difficult to autofocus when the aperture is less than F 8. For this reason, most SLR cameras have autofocus auxiliary light (Autofocus aidlights) emitters that emit beams with infrared stripes to help automatically focus subjects of different textures. These auxiliary lights do not work when the light is bright enough. When using, it should be noted that because the active launch window is on the right side of the camera, do not let your hand block the launch window when holding the camera. It doesn't focus when blocking the launch window. There is no launch window for emitting auxiliary light on the fuselage of a professional camera, only a flash is installed, and the emission window on the lamp is used to emit auxiliary light for active focus. 


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