Depth of focus — area before and behind focus point rendered sharp. Small aperture (high f-number) = deep DoF; large aperture (low f-number) = shallow DoF for isolation.
The depth of field determines what your camera renders sharply and what becomes blurred. On set, it works like this: You focus on a specific point – an actor's eye, an object in hand – and everything in front of and behind it becomes more or less out of focus depending on the aperture. A small aperture number (f/1.4, f/2.8) creates shallow depth of field, a large aperture number (f/16, f/22) makes almost everything sharp. This is not a mistake, but your creative decision.
In practice, you combine three parameters: focal length (wide-angle = deep depth of field, telephoto = shallow depth of field), aperture (the obvious tool), and distance to the subject (closer = shallower). If you shoot a close-up with f/2.8 on an 85mm lens, the depth of field is centimeter-thin – nose sharp, ear already soft. This is your isolation shot. Conversely: a landscape shot with f/11 on a 24mm lens, and the entire horizon is sharp in front of you. No distraction by blur.
The trick lies in focus pulling. With extreme shallowness (f/1.2) over long takes, you have to pull constantly – follow the focus point. This is craftsmanship that needs training. Modern cameras with autofocus tracking help, but on set, you rely on the 1st AC (Focus Puller), who works with a measuring device or live view. With deep depth of field, you have leeway – the focus holds longer, even with movement.
Psychologically, it's powerful: shallow depth of field isolates, draws the eye, creates intimacy – a classic tool for portraits and emotional moments. Deep depth of field creates context, spatial presence, is more authentic, more documentary. Some cinematographers consciously work with very shallow depth, others reject it and prefer spatial clarity. Both are legitimate, it's style. Understanding all three parameters together – that's the toolkit. Without understanding the balance, it becomes random.