Subject Distance for Video: Staying in Focus While Moving

A still photographer can miss a frame and take another. In video, a subject sliding out of focus for half a second is visible, and the cause is mostly arithmetic: how far your subject moves compared with how deep your zone of focus is.

Why close and long is the hard combination

Depth of field grows roughly with the square of the distance and shrinks with the square of the focal length, at a given aperture. A subject at 2 m has about a quarter of the depth of one at 4 m. A lens three times as long has about a ninth of the depth. Here is what that looks like on full frame:

The same applies to the focus movement needed. A 10 cm step toward the camera changes the distance by 10 percent at 1 m and by 1 percent at 10 m, so close subjects demand far larger corrections for the same move. Telephoto, close and wide open is the toughest combination for continuous autofocus. Our video versus photo DOF post on the sister site looks at the same effect from the depth side.

How much depth is enough?

Only movement toward or away from the camera matters. Someone walking across the frame stays at the same distance and stays in focus. A person walking toward you covers about 1.4 m a second, which is 28 cm in the fifth of a second a camera may take to catch up. So a zone of 20 to 30 cm is the least you want if the subject is moving along the lens axis. The 85 mm f/2.8 at 3 m is borderline, and the 85 mm f/1.8 at 2 m is only workable with excellent tracking or a subject who stays put.

Matching lens and distance to the job

Run-and-gun and documentary work is the hardest to predict, because your subject's distance changes all the time. A 24 to 35 mm lens at f/2.8 to f/4, 1.5 to 3 m away, keeps most of the movement inside the focus zone. A 35 mm at f/4 from 2.5 m gives about 1.2 m. Telephoto from close range, or f/1.4 to f/1.8 on a moving person, is asking for trouble.

Interviews are the forgiving case, since the subject stays put and only leans a little. A 50 mm at f/2.8 from 2.5 m gives about 40 cm, which absorbs leaning easily. For a more cinematic look, an 85 mm at f/2.8 from 4 m gives about 35 cm and works well with eye autofocus.

On controlled sets it is common to pull focus by hand. Mark the focus distance for each position the subject will occupy with a tape measure, then pull between those marks as they move. Cinema lenses have a distance scale on the barrel, which makes this easy. Stills lenses usually do not, which is why follow-focus gear exists.

Wildlife is a long-lens problem at longer distances. A 300 mm lens at f/5.6 on a bird 8 m away gives about 22 cm, and a 400 mm lens at f/5.6 at 15 m gives about 44 cm. That is enough for tracking autofocus to cope with a hopping bird, but only just, and subject-tracking AF with animal detection does most of the work.

Autofocus settings that help

Use face, eye or subject detection rather than a fixed area when you are working alone, because it is more stable once it has locked on. Look in the video menu for a setting that controls how quickly focus moves and how readily it switches subject. Names vary by brand, "AF transition speed" and "subject shift sensitivity" on Sony for instance. A slower, less eager setting looks calmer on screen. A high sensitivity makes the lens keep micro-adjusting, and you see it as hunting.

Lenses matter too. Stepping or linear focus motors, labelled STM, stepping motor or similar, are quieter and smoother than older designs. Some lenses also change their field of view slightly as focus moves, called focus breathing, which shows in video. Some cameras can compensate, usually only with particular lenses, so check yours.

Marks and blocking

On any planned shot, decide where the subject will stand and move, and tell them. For a 50 mm at f/2.8 from 3 m, the zone runs from about 2.7 to 3.3 m, which is roughly half a metre. "Stay within an arm's length of where you are" is a perfectly normal instruction, and people manage it easily once they know why. Rehearse the move once and you will see whether the focus follows.

To check any lens, aperture and distance for your own shot, use the calculator, and measure the subject's height in a test frame to confirm how far away you really are.

Related reading: Portrait Distance: What Focal Length at What Range, Wildlife Safety Distances.