Shallow depth of field in documentary, and when to stop down
What controls shallow depth of field, how sensor size and equivalence change it, and why documentary shooters often stop down to keep focus and context. Some links here are affiliate links.
In this piece (8 parts)
The most beautiful shot I ever lost was at f/1.4. An elderly man at his kitchen table in Burnaby was telling a story about his first winter in Canada, and at the punchline he leaned forward and laughed. The shallow depth of field I had been so pleased with an hour earlier did exactly what physics says it will: his eyes went soft and his ear stayed sharp. The laugh lasted four seconds. I could not use any of them.
Shallow focus is the look everyone wants from a new camera, and in documentary it is a tool with real costs. This guide covers what controls depth of field, how sensor size and equivalence change the numbers, and when to open up or stop down.
What depth of field means
Depth of field is the zone in front of and behind the point of focus that still looks sharp. Focus does not stop at an edge; it fades. "Acceptably sharp" depends on how large the picture will be seen, which is why lens charts work from an agreed blur limit called the circle of confusion. A shallow depth of field puts a thin slice of the scene in focus and lets everything else melt. A deep one keeps most of the frame sharp.
Stills photographers and filmmakers use the same optics, so everything you know from depth of field photography carries over. The difference is that our subjects move during the shot, and they rarely tell us when.
The four things that control it
- Aperture: a wider aperture, meaning a smaller f-number, gives shallower depth of field. Each time you open up by two stops (f/5.6 to f/2.8, say), the zone of focus roughly halves at normal working distances.
- Distance: the closer you are to the subject, the shallower the zone. In practice this is the strongest control of the four, and the one people forget.
- Focal length: from the same spot, a longer lens gives shallower focus. Keep the framing the same, though, by stepping back with the longer lens, and the depth of field barely changes. What changes is the background: the longer lens magnifies it, so it looks blurrier and simpler.
- Sensor size: for the same framing and the same f-number, a bigger sensor gives shallower focus, because you need a longer lens or a closer position to fill the frame.
Here is what that looks like in numbers, worked out with the standard formulas for a full-frame camera and a 50 mm lens focused on a person two metres away (circle of confusion 0.03 mm, the common full-frame figure).
| Aperture | Sharp from | Sharp to | Total depth of field |
|---|---|---|---|
| f/1.4 | 1.94 m | 2.07 m | about 13 cm |
| f/2.8 | 1.88 m | 2.14 m | about 26 cm |
| f/5.6 | 1.77 m | 2.30 m | about 53 cm |
| f/11 | 1.59 m | 2.69 m | about 1.1 m |
Now go tighter. An 85 mm lens at f/1.8, focused on a face 1.5 metres away, holds a zone of roughly three centimetres. A person leaning back in a chair moves further than that between two sentences.
Sensor size and equivalence
Crop factor compares a sensor's diagonal with the 36 by 24 mm full frame. Using the typical sizes in the Kit Room, APS-C from most makers works out at about 1.53, Canon's APS-C at 1.61, Micro Four Thirds at 2.0 and Super 16 at nearly 3.
Multiply both the focal length and the f-number by the crop factor and you get the full-frame lens that gives the same framing and the same depth of field. A 25 mm lens at f/1.4 on Micro Four Thirds frames like a 50 mm on full frame and gives about the same depth of field as f/2.8 there. Run the formulas and the two come out within a centimetre of each other at two metres. The crop factor calculator does the arithmetic for any sensor and lens.
One thing does not change: exposure. f/1.4 lets in the same light per square millimetre on every sensor, so your shutter and ISO stay the same. Equivalence is about framing and focus, not brightness.
This is also why phones struggle to give a truly shallow look. Their sensors are tiny, so the depth of field is deep by nature, and the blurred backgrounds they offer are produced in software, which tends to stumble around hair, glasses and the edges of hands.
When shallow focus helps
It cleans up chaos. A busy office, a cluttered garage or a crowd of passers-by can turn into soft shapes, and the face you care about stands clear of them. In interviews, a gently soft background separates the subject from the wall without a back light doing all the work.
It directs attention in detail shots: hands on a loom, a stack of letters, a medal in a palm. On a tripod with a subject who stays still, open up as far as you like.
It can also protect people. When the background holds bystanders who have not agreed to be filmed, or a street sign that would identify a location, softening it is a legitimate tool. And in a dim room, the widest aperture is sometimes simply the only way to get an exposure.
When shallow focus hurts documentary
Unscripted moments are where it fails. People shift, lean, stand up and turn without warning. At f/1.4 with no focus puller, a camera operator who is also framing, listening and following the conversation will miss focus, and autofocus will sometimes jump to the wrong face or the background. The best moment in a scene is often the one nobody saw coming, which is precisely the moment a paper-thin focus zone is most likely to miss.
It also strips away context. In documentary the background is evidence: the room someone lives in, the people standing at the edge of a meeting, what is pinned on the wall, the state of a kitchen. Blur all of it and every place turns into the same creamy nowhere. I find that look has become so familiar from corporate and wedding video that it reads as a style rather than a choice.
Two people in frame at different distances will not both be sharp at f/1.4, and in a conversation scene that is often the whole shot. The observational tradition knew this. The direct cinema crews who walked into events with wide lenses held close worked on 16 mm, a small frame that, with those lenses, kept most of a scene in focus. It let them follow life instead of chasing it with a focus ring.
When to stop down
A few working rules, all in full-frame terms (divide by your crop factor for the setting on your own camera):
- Observational scenes with movement: f/4 to f/8. Let a wider lens and a closer position give you intimacy instead of a thin focus zone.
- Seated interviews: f/2.8 to f/4 leaves room for a lean while still softening the background.
- Two people in frame: stop down until both faces sit in the zone, or seat them at the same distance from the lens.
- Detail shots and locked-off B-roll: open up freely.
- Dim rooms: add light rather than opening up. A soft key from the interview lighting kit buys you a stop or two of depth.
- Always: switch on focus peaking or magnification, and check focus on playback before the subject leaves.
ND filters for shooting wide open outdoors
In daylight, a wide aperture lets in far too much light. You could raise the shutter speed to compensate, but at 1/1000 every movement turns choppy and staccato, a look that rarely suits documentary. The usual fix is a neutral density filter, which darkens the whole image evenly without changing its colour.
The old sunny sixteen rule gives a sense of scale. In bright sun at ISO 100 and a 1/50 shutter, correct exposure sits around f/22, so opening up to f/2 means about seven stops too much light. If your camera's base ISO is 800, add three more. Filters are labelled by optical density (ND 0.3 is one stop, 0.6 is two, 0.9 is three) or by factor (ND8 is three stops, ND64 is six).
Variable NDs turn a ring to change strength, which is handy when you move between sun and shade. At their strongest settings, some produce a dark cross pattern or a colour cast, so stay within the range the maker recommends. Fixed filters are cleaner. Many video cameras have ND built in, which is the most convenient of all. Whatever you use, avoid stacking several filters on a wide lens, where they can darken the corners.
Questions
What is depth of field?
The zone in front of and behind the point of focus that looks acceptably sharp. Shallow depth of field keeps a thin slice sharp and blurs the rest; deep depth of field keeps most of the scene sharp.
How do I get shallow depth of field with a small sensor?
Use the widest aperture you have, get closer to the subject, put more distance between subject and background, and choose a longer focal length. A fast lens helps most, because a small sensor needs a smaller f-number to match a full-frame look.
Does a longer lens give shallower depth of field?
From the same position, yes. If you step back to keep the same framing, depth of field stays about the same, but the background is magnified and looks blurrier.
What aperture should I use for documentary interviews?
In full-frame terms, f/2.8 to f/4 is a sensible range: the background softens, and a subject who leans back stays in focus. Divide by the crop factor to find the matching setting on a smaller sensor.
Do I need an ND filter to shoot wide open outside?
Usually yes. Without one, bright daylight forces a very fast shutter that makes motion look choppy. Built-in ND, a fixed filter or a good variable ND all work.
