Totalostframe

Hi everyone, I use a Canon 5D mark III for shooting video it has manual focus and I also use a gimbal zhiyun crane 2 but I just don’t know how to keep the subject in focus when I’d like to use wide aperture 2.8 and the subject comes toward to me or gets further away from me. I’m planning to buy a field monitor as I cannot see if the person walks out of focus or not. anyone else how you solve this issue during shooting?