What does focal length mean in Blender? I know how lenses work, and I know what a focal point is, but I don't get what Blender means by "focal length".

I can set the distance to the focal point of a camera in the depth of field panel, while changing the focal length in the lens panel changes my FOV, (larger focal length -> smaller FOV) but what quantity is this change based on?
I need to set the FOV with great accuracy, based on distances calculated from the geometry in my scene, so I would like to know what the meaning is of the distance that I have to fill in in the lens panel.
Focal length is the distance between the lens and the focal point:



At focal point the image is sharp and it's the place where the camera sensor is located (or camera film). The Field of View of each lens depends on the sensor size:



The conversion formula is this:

FOV = 2 * arctan(half_the_diagonal_of_sensor / focal_length)


Blender allows you to enter the "zoom" of camera in either Focal length (mm) or in Field of View (°):