Measurable distance from nearest to farthest acceptable sharp point — critical for follow-focus and rack moves. Calculated via hyperfocal distance and circle of confusion.
On set, focus isn't binary — either sharp or out of focus. In between lies a zone where the eye still perceives sharpness. This depth range is the measurable distance between the nearest and furthest point that is still rendered acceptably sharp. For the focus puller, this isn't academic — it's the working zone. The narrower this range, the more precise the pulling needs to be. The wider it is, the more leeway for follow focus.
The size of the depth range depends on three factors: aperture, focal length, and distance to the plane of focus. A wide-open aperture (f/1.4) makes the range extremely small — often just a few centimeters at normal distances. This makes it easy to be out of focus quickly during a portrait on set. A stopped-down aperture (f/5.6, f/8) extends the range considerably — suddenly you have room to work. The hyperfocal distance is a tool here: focusing at a specific distance and using it correctly extends the depth range maximally and symmetrically. This is the secret weapon for static focus without follow focus.
Longer focal lengths (80mm, 150mm) compress the depth range — particularly problematic for close-ups. Wide-angle lenses (24mm, 35mm) expand it. This is no coincidence: this is why primes are used instead of zooms when focus is dramatically important. The circle of confusion — the acceptable blur that the eye still reads as sharp — is typically 25–40 micrometers on the sensor, often calculated more conservatively in cinema.
In practice: For wide shots at f/2.8 with a 50mm lens at 2 meters, you have a good 80–100 centimeters of depth range — relaxed for dialogue. At 100mm at 1.5 meters and f/1.8? Barely 10 centimeters. Every millimeter becomes a drama, and follow focus is indispensable. Modern digital metering tools and focus peaking help visualize this range on the monitor. On film, it required old depth-of-field tables and experience. Both still work today.