I put a 90mm prime [1] on my Sony, set it to aperture priority, put the strap over someone's head and deputize them to get headshots ("frame it up with the viewfinder and push the button") and they do OK so long as the light is predictable. I wish I could tell the auto mode to let the ISO go higher than it will because I do noise reduction in developing such that there is no real quality loss at 6400.
[1] takes lovely portraits and no focus to deal with
[1] takes lovely portraits and no focus to deal with