In photography: zero in on = move in closer to something (in order to get as close a shot as possible of it) focus on = concentrate on a particular detail (and get as sharp an image as possible of it) Both can also be used figuratively, in which case they mean much the same as each other – although to focus on something is more general and much more common. You could use zero in intransitively, but in the vast majority of cases it’s used in relation to something specific, in which case it needs the preposition “on”.

All the answers are correct, but I want to note that "zero in on" usually implies a much more narrow scope than "focus on." eg: "When the plane crashed, the investigators initially focused on the maintenance records of the plane. When they learned more, they zeroed in on the bad repair of the rudder as the cause of the accident." Also, one would say, "in order to pass the test, you should focus on studying grammar." I don't think I have ever heard anyone say "you should zero in on studying grammar."

Hi Tura In the UK, we say 'focus on'. To 'zero in on' is American English. Victoria