Hi. I have a brand new SONY 20mm f1.8 lens. SEL20F18G, attached to an A7iii.  When in continuous focus, the image on screen jumps in and out considerably as the autofocus is moving around. Is this normal? It 'zooms' in and out quite a lot as the internal mechanism moves forwards and backwards trying to find the focus. I have never seen it before on a SONY lens (I have the 55mm prime). It does not occur when the camera is in Auto, but is quite pronounced when it is in AP mode.