I looked at the Apple’s website and noticed that they were giving focal lengths for each camera but no sensor size. On one of their camera they give the focal length and field of view. So I took my P-Cam app (a cinematographer app to do camera and lenses math) and for a camera to have a focal length of 13mm and a field of view of 120°, the sensor size should be about the size of film IMAX 65mm cinema camera. 13mm on a 35mm still camera would give an 80° field of view and you would need a 10.39mm lens to get to 120° and a 1mm focal length on a 1.4” sensor.
So how are they measuring those focal lengths?
Field of view has to do with the curvature of the lens, not size. How do you think 360 cameras work? Not with a massive sensor, that’s for sure.
No, field of view is the maximum angle you can cover if you were to trace the cone of what is inside the frame of the sensor/film plan for a given focal length. Focal length is the distance between the nodal point and the film plan/sensor. The nodal point is the point where the light rays all pass through when crossing towards the sensor. For lenses that have multiple elements you still have the same rules as you use the focal point of all the elements cumulated. There is no curvature of a lens.