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.