What does the focal length of the lens mean?

Lens focal length refers to the range of focal length of zoom lens.

The focal length of the lens is divided as follows:

1, standard lens: its viewing angle is about 43 degrees, and the viewing angle of the photo is closest to the human eye;

2. Frame: Different frames have different standard lens focal lengths. For example, the standard lens focal length of "135" film is about 50 mm, and that of "120" film is about 80 mm. Most documentary photographers like to use standard lenses, such as French photographer Bresson.