I have watched many Youtube vids on menu setting up the a7R iv. Many recommend a custom back button to trigger AF and therefore to disable the half-press-shutter function for AF. Does holding down back-button lock the AF, whereas the shutter releases AF during the 10-frame burst? I can’t detect any difference when switching back and forth between AF-ON back button and shutter half- depress. But still the seasoned shooters like the custom back button. Do we know why?