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What Is Depth of Field? A Cinematographer’s Explanation
Depth of field is the distance between the nearest and farthest things in a shot that look acceptably sharp. Everything outside it falls off into blur.
Also called: DOF · focus falloff
What changes it
Open the iris and the field gets shallower. Put on a longer lens and it shallows again. Focus closer and it shallows a third time, which is why a tight close-up on a moving actor is the hardest thing a focus puller does.
Sensor size sets the baseline. A larger format at the same framing and stop gives you less depth than a smaller one. The difference is that you choose the format once, when you pick the camera, and the other three are live decisions on the day.
Circle of confusion
Sharpness does not stop at a line, it falls off. The circle of confusion is the agreed threshold: the largest blur spot that still reads as a point at normal viewing size. Every depth of field figure depends on the value assumed for that format, which is why two calculators can hand you different numbers for the same lens at the same stop.
Why anyone works it out on set
A focus puller needs to know how much room a move leaves before the actor goes soft. A 50mm at T2 on full frame, focused at eight feet, gives you roughly seven inches of usable depth. Knowing that before the take is the difference between a slate that holds and one you go again on.
