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Active and passive focus of an auto focus camera

The camera is a device that records and captures images, and focusing is a very important function in the camera, which determines the clarity and focus accuracy of the image. In the field of photography and videography, focusing can be divided into automatic focusing and manual focusing. Autofocus can be further divided into active focus and passive focus. In this paper, the principle and function of active and passive focus of auto focus camera will be introduced in detail.




Active auto focus

Active focus means that the camera actively senses and adjusts the focus through the built-in autofocus system to ensure that the captured image is clear. It uses a number of sensors and algorithms to determine the focus position by analyzing contrast, sharpness and edge information in the image. Once the focus is determined, the camera automatically adjusts the focal length and position of the lens to achieve the best focus effect. An active focusing system usually consists of one or more sensors and an autofocus algorithm.


In active focusing systems, the most common sensor is the phase focusing sensor. It determines the focus position by measuring the phase difference of different light rays on the lens. When the phase difference is the smallest, the phase focus sensor will get the sharpest image. This sensor responds very quickly to light and is suitable for fast-moving subjects. Phase focus sensors are usually integrated into the photosensitive elements of the camera and can accurately measure the focus position and pass this information to the autofocus algorithm.


Another common type of sensor is the contrast sensor. Contrast sensors determine focus positions by analyzing contrast changes in different areas of the image. When the image contrast is maximized, the contrast sensor will get the sharpest image. The contrast sensor is relatively slow in response compared to the phase focus sensor and is suitable for still shooting or scenes with relatively small focus adjustment. Contrast sensors are also often integrated into the photosensitive elements of the camera to quickly and accurately measure the focus position.


Active and passive focus of an auto focus camera - Sony Camera Information - 2


Autofocus algorithm is the core part of active focusing system. Based on the input from the sensor, it analyzes the contrast, sharpness and edge information in the image to determine the best focus position. Autofocus algorithms usually include a series of operations and judgment processes to find the optimal solution of the focus position. These algorithms usually have a certain adaptability and can be adjusted and optimized according to different scenes and subjects.


The active focus of autofocus cameras is mainly used in video conferencing and distance education, security monitoring, medical imaging, drones and robots.


Passive focusing

Passive focus means that the camera triggers the focus function by pressing the shutter button or other operations in the auto focus mode according to the needs of the user and the changes in the shooting environment. The difference between passive focusing and active focusing is that passive focusing does not actively perceive the focus position, but determines the focus position through the user’s actions. In the passive focus mode, the camera will maintain the stability of the focus position according to the user’s requirements, and automatically adjust the focus length and position during the shooting process to ensure the clarity and focus accuracy of the image.


application of passive focusing


The active focus of auto-focus camera is widely used in sports shooting, documentary, portrait photography, advertising and commercial photography, street photography and other fields.


To sum up, the active focus and passive focus of an autofocus camera are achieved by different sensors and algorithms. Active focusing Through the built-in autofocus system, the focus is actively sensed and adjusted to ensure clarity and focus accuracy of the image. Passive focus is the automatic focus mode, through the user’s operation to trigger the focus function to meet the needs of users and changes in the shooting environment. Whether active or passive focusing, they bring a more convenient and flexible shooting experience to the camera, improving the quality and accuracy of the image.



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