As you can see, the value shown in the AVCaptureSystemZoomSlider is not the same as the raw camera zoom factor.
I tried to calculate this value, and it seems it's 0.8. (5-1)*0.8=4.2-1 in this image.
It seems this factor only applies to the default wide-angle camera. And I can't get this value from anywhere. (It's not displayVideoZoomFactorMultiplier btw, I checked that.)
What is it?