Woah. See this is the problem with me having an iPhone for too long. Never knew this kind of stuff existed. hahaYou can do it that way but in pro mode if you touch and hold the screen with spot metering selected it will show a little locked circle for AF/AE then use manual focus if needed to capture your wife but exposure is set on different object
Try it indoors say set exposure based on table lamp, then pan to a different subject