What's the difference between camera focusing and focusing adjustment?

Zoom: it is to change the distance between the lens (optical center) and the basement membrane, that is, the image distance. Only by changing the image distance can we change the shooting angle, that is, we can bring the subject closer or farther away. So when you zoom, you can see the lens stretching or shrinking.

Focusing: Focusing is to change the focal length of the lens so that objects at different distances can focus on the basement membrane to form a clear image. The change of the focal length of the lens is realized by the change of the lens inside the lens, and the distance between the optical center and the basement membrane remains unchanged, so the lens will not become longer or shorter.

The focal length is related to the viewing angle of the picture. The longer the focal length, the narrower the viewing angle and the closer the picture looks. The shorter the focal length, the wider the viewing angle, and the more things the picture can hold. Focusing is related to the distance of the subject. The focal length of the lens can be adjusted between the nearest point and the farthest point in the lens focusing range, that is, infinity. The closer the focal length is to the actual distance of the subject, the clearer the picture will be.

The order of taking pictures is to adjust the focal length (zoom to get a suitable picture), then focus, and finally determine the exposure (set the aperture, shutter and sensitivity together).

The traditional camera expands the data and realizes autofocus in a way similar to visual ranging. The camera emits an infrared ray (or other rays), determines the distance of the photographed object according to the reflection of the photographed object, and then adjusts the lens combination according to the measurement result to realize automatic focusing.

This automatic focusing method-direct, fast, easy to realize and low cost-sometimes makes mistakes (when there is something else between the camera and the subject, such as glass, or the light is insufficient, automatic focusing cannot be realized), and the accuracy is poor. Today's high-end cameras generally don't use this method.

Because the camera actively emits rays, it is called active, and because it is actually only ranging, it is also called non-TTL because it does not judge whether coking is correct through the actual imaging of the lens.

Compared with active autofocus, passive autofocus was developed later, that is, judging whether coking is correct according to the actual imaging of the lens. The basis of judgment is generally contrast detection, and the specific principle is quite complicated. Because this method is realized by lens imaging, it is called TTL autofocus.

It is precisely because this autofocus method is based on lens imaging, which has high focusing accuracy and low error rate, but the technology is complex, the speed is slow (except for the advanced autofocus lens using ultrasonic motor) and the cost is high.

References:

Focus-Baidu Encyclopedia