There can some misconception when it comes to focal length of a camera lens but it is quite simple to understand it, the focal length is simply what the makers of the lens say it is, it cannot be changed. The field of view is another term that gets misused, some confuse with the angle of view, although the two are very different, one is a bit varying while the other seems to be less varying. In this article, I will be explaining these terms in very simple languages avoiding the math and physics involved.
The field of view is everything your camera with the lens can see and capture. The field of view is not necessarily what you see in the viewfinder as some cameras viewfinder does not have 100 percent coverage, so what you can see in your viewfinder is, in fact, less than the actual field of view. The angle of view, on the other hand, is the what your camera lens can see in different degrees, some cameras can shoot above 60 degrees and that will mean more coverage from an angular view.
The major difference between the field of view and the angle of view is that the latter is dependent on both the camera and the lens mounted on it while the former id dependent on the just lens and has the camera has no effect on it. The angle of view produced by a lens will differ on different camera but the field of view produced by a lens is universal irrespective of the type or specification of the camera it is mounted on.
The focal length is a characteristic of a camera lens and every camera lens has its own focal length which is usually measured in millimeter. The focal length of a lens is not subjected to the camera the lens is mounted on and the focal length will remain the same irrespective of the configuration of the camera.
There is a relationship between the focal length and field of view of a lens, it is termed the equivalent focal length, for example, the 70mm-400mm has an equivalent focal length of 70 (the math is not correct, just a statement to explain equivalent focal length). Now, the focal length of the lens does not change but the field of view can with some adjustment to the lens used in the shooting and the sensors on the camera.