What does the focal length in the camera mean?

The concepts of "focal length" and "focal length" in photography are often confused.

As the above two people said, the distance from the midpoint of a lens or the theoretical point of a lens group to the focal plane (that is, film or photoreceptor) is the so-called "focal length", which is optically called the image distance (not the object distance mentioned upstairs).

The distance from the object to the midpoint mentioned above is the optical object distance. Under normal circumstances, the "focal length" should be called this object distance, but in order to use the distance from the focal plane to the object conveniently when marking the lens.

The actual function of focal length (the principle is omitted here, and you can check the encyclopedia if you are interested) is reflected in the lenses with different focal lengths, and the refraction angle of light is different, so the viewing angle reflected in the photos is also different. The shorter the focal length, the wider the viewing angle, the longer the focal length and the narrower the viewing angle.

In addition, if the size of the subject remains the same, the perspective effect of the lens with different focal lengths is different. Because of the wide viewing angle, the lens with the focal length needs to be closer to the subject, and the object behind it will appear smaller and farther away from the subject. On the contrary, the telephoto lens needs to be farther away from the subject because of its narrow viewing angle, in order to obtain the subject of the same size, and the thing behind the subject will be bigger and look closer to the subject. This detailed truth needs drawing to illustrate, so I won't elaborate on it. I tried it a few times before I realized it.