basic concept
focus: The lens of the camera is a set of lenses. When parallel rays pass through the lens, they will converge to a point, which is called the focus.
focal length: The distance from the optical center (the center of the convex lens) to the focal point, denoted by "f".
fixed focus lens: A lens with a fixed focal length, that is, a fixed-focus lens.
zoom lens: A lens whose focal length can be adjusted and changed, that is, a zoom lens.
focus
The process of adjusting the camera to make the subject into a clear image is the process of focusing (focusing).
Focusing is also called focusing. It is not changing the focal length of the lens; it is changing the image distance, adjusting the distance between the imaging surface and the lens, so that the distance from the imaging surface to the optical center is equal to the image distance, so that the object can be clearly imaged on the photosensitive element. The imaging position is outside the focal length of the lens and within the focal length of the lens, and the imaging position is where the photosensitive element (CCD/CMOS) is located, and the imaging is very clear at this time.
zoom
Zooming changes the focal length of the lens, resulting in changes in the viewing angle or image size, to achieve the effect of zooming in or zooming out
This is usually achieved through changes in lens composition. The longer the focal length, the narrower the angle of view, the fewer scenes that can be tolerated in the picture, and the closer the picture looks; the shorter the focal length, the larger the angle of view, the more scenes that can be accommodated in the picture, and the picture looks farther away.
The general process is to zoom first and then focus
In the general process of taking pictures, the distance between the object and the camera is not always the same. For example, when taking pictures of people, sometimes you want to take pictures of the whole body, and the distance is far away; Yes, if you want to take a clear image like this, you must change it with the object distanceDistance from photosensitive element to optical center（focal length f), then focus so that the subject is in focus.