Depth of field

Three things control it: aperture, focal length, and how close you are to the subject. Open the aperture to f/1.8, use a longer lens or step nearer, and the background melts away. Stop down to f/8 and the room behind you comes back. Talking-head channels reach for shallow focus because it separates the presenter from a messy room, though it also means a small lean forward can leave your eyes soft. Anything where the viewer needs to see the surroundings as well as you, a workshop, a whiteboard, a game setup, wants more of the frame sharp.