Focused zone between nearest and farthest sharp point — controlled by aperture, focal length, distance. Narrow DOD isolates subject; wide DOD holds everything sharp.
You're facing a classic situation: the actor is three meters away, with a wall of posters behind them. You want them sharp, but the background completely dissolved. Or vice versa – an establishing shot where the foreground to background should be tack sharp. All of this is governed by the depth of field, this invisible area between the nearest and farthest point that still appears acceptably sharp.
Practically, it works like this: your aperture is the first adjustment. At f/1.4 or f/2.0, the range becomes extremely narrow – only a few centimeters of depth of field. This allows for aggressive subject isolation: eye sharp, ear already out of focus. You see this constantly in modern portraits or close-ups. If you set it to f/8 or f/11, this range expands considerably. Suddenly, you have two to three meters of depth of field. At f/16, almost everything is in focus, provided the distances are right.
Focal length works against you: a 50mm lens gives you more depth of field at the same aperture than a 135mm. The 135mm has a shallow depth of field – it's perfect for subject isolation, but difficult for complex scenes where multiple actors are at different planes. The 24mm gives you wide-angle depth of field – even at f/2.8, much remains sharp. You notice this quickly when switching between lenses.
Your shooting distance is the third variable. The closer you are, the shallower the depth of field becomes. Macro photography live: at a 10cm distance and f/4, the sharp area is sometimes only 2mm wide. This is a technical challenge. At a 5-meter distance with the same aperture and focal length, you suddenly have meters of depth of field. Many older films deliberately use this – they work with longer distances and moderate apertures to have more control and not have to constantly pull focus.
On set, you always calculate the depth of field beforehand. With a DOF app or classically: enter aperture, focal length, focus distance, and you'll know exactly where your front and back limits lie. This is essential for camera moves – as you approach the subject, the range narrows. That's why a follow-focus puller is used for a narrow range. With a large range (f/11, wide-angle, greater distance), you can move more freely.