Camera current and exact fieldOfView

I can get camera fieldOfView using AVCaptureDevice.activeFormat.videoFieldOfView and this gives me 58.63214874267578 on iphone 6s. So far so good, but this is actually not accurate enough for me. Now lets see how iPhone camera preview works - when you point it at distant object, it moves lens a little bit to catch focus. But then actuall field of view changes a little - when camera is catching focus it looks like it was zooming in and out a little. And that is fine, I need camera preview to be sharp, but when this happends, real field of view changes a bit, it goes to over 61 degrees even (because lens has moved). I need to know how to get or calculate this exact field of view for current camera frame. This is kinda what ARKit does (I think), because when you see it catches sharpness it resizes a little camera preview and AR content with it. Does anybody have an idea how to calculate this field of view value for current camera lens position (or focus position)?