https://en.wikipedia.org/wiki/Focal_length
https://photographylife.com/what-is-focal-length-in-photography
The focal length of an optical system is a measure of how strongly the system converges or diverges light.
Without getting into an in-depth physics discussion, the focal length of a lens is an optical property of the lens.
The exact definition is: Focal length measures the distance, in millimeters, between the “nodal point” of the lens and the camera’s sensor.
Lenses are named by their focal length. You can find this information on the barrel of the lens, and almost every camera lens ever made will prominently display the focal length. For example, a 50mm lens has a focal length of 50 millimeters.
In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view;
Conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view.
On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.
Focal length is important because it relates to the field of view of a lens – that is, how much of the scene you’ll capture. It also explains how large or small a subject in your photo will appear.
