How much is in focus — and how much is in frame?

Three questions, one set of inputs

A 1st AC wants to know how much room the actor has before the eyes go soft. A DoP on a recce wants to know which lens gives a medium close-up when the wall is 2.5 m behind the camera. A director wants to know whether the 35 still feels wide on this sensor. All three come from the same four numbers: sensor size, focal length, stop and distance.

The Depth of field view draws the shot from above, to scale: the angle of view as a wedge, the focus plane, and the band that stays acceptably sharp. Below it, the whole lens set at your stop and distance — click a row to switch lenses. Field of view shows the frame itself with a 1.80 m person at the focus distance, eyes on the upper third, and names the shot size. Which lens? works backwards from the shot size you want.

How we calculate

Depth of field uses the standard thin-lens formulas. The hyperfocal distance is H = f² ÷ (N × c) + f; the near limit is s(H − f) ÷ (H + s − 2f) and the far limit s(H − f) ÷ (H − s), infinite once you focus at or beyond H. The stop is treated as the f-number; with cine lenses you enter the T-stop, which is usually within a third of a stop of it.

The circle of confusion c is the blur spot still accepted as a point. The cinema convention is roughly sensor width ÷ 1000 — the classic 0.025 mm for Super 35 on Angénieux charts, 0.028 mm for an Alexa 35, 0.036 mm for full frame. Still-photo tables often use 0.030 mm for full frame (diagonal ÷ 1442), which is why apps disagree. Switch to strict (÷ 1500) when focus will be judged on a large 4K screen.

Field of view is 2 × atan(width ÷ 2f), taken from the image area of your delivery format: a 16:9 frame from the full sensor width, cropped at the sides when the sensor is not tall enough. Shot sizes are defined by how much of a standing 1.80 m person is visible: up to 25 cm frame height is an extreme close-up, up to 45 cm a close-up, 75 cm a medium close-up, 1.10 m a medium shot, 1.60 m a medium long shot, 2.60 m a full shot.

H = f² ÷ (N × c) + f near = s(H − f) ÷ (H + s − 2f) far = s(H − f) ÷ (H − s) (∞ if s ≥ H) FoV = 2 × atan(w ÷ 2f) frame width at s = s × w ÷ f

Where the tool is wrong

• Sharpness is not an edge. The near and far limits mark where blur crosses an agreed threshold. Just inside the band, eyes are already softer than on the focus plane; on a 10 m screen, the threshold moves. Treat the band as the room for error, not as the area that looks crisp.
• Recording modes. The camera list uses each sensor’s full active area. Many cameras crop for higher frame rates or certain resolutions — then the angle of view gets narrower. Enter the active area of that mode as a custom sensor size.
• Close focus and macro. The formulas assume the subject is far away compared with the focal length. Below about ten focal lengths (0.85 m for an 85 mm) they flatter the depth of field; for inserts and product macro, test on the monitor.
• Anamorphic lenses. A 2× anamorphic squeezes the horizontal angle: enter the horizontal angle of the de-squeezed image by halving the focal length for the field of view, but use the real focal length for depth of field.
• Breathing and real focal lengths. Engraved focal lengths are nominal, and many lenses change their angle of view as you pull focus. At close distances the true frame can be several per cent tighter or wider.

From the set

An interview on an Alexa 35 with an 85 mm at T2, the subject 1.8 m away: the frame is about 59 × 33 cm, a close-up, and the depth of field is 4.8 cm — 2.4 cm in front of the eyes, 2.4 cm behind. A guest who leans forward while making a point is out of focus. At T2.8 it is 6.8 cm, which most focus pullers can hold without a marked chair.

The same frame on an Alexa Mini LF needs about 111 mm for the larger sensor, and at T2 the depth of field drops to 3.6 cm. To get the Alexa 35 look back you stop down to roughly T2.8 — about 0.8 of a stop for the 1.31× larger sensor. That is the honest version of “large format has shallower depth of field”: only at the same stop, and you can buy it back.