Exposure for video — aperture, shutter, ISO and native ISO
In video you have one less free lever — the shutter is tied to the frame rate. So light is controlled by the aperture, ND filters and only finally by ISO.
Three levers, each with a side effect
The amount of light that reaches the sensor depends on two things: the aperture (how much light the lens lets through) and the shutter speed (for how long). The third lever, ISO, doesn’t add light — it changes what the camera does with the signal it has gathered. Each lever has a side effect, and it is the side effect that decides which one you use.
DIAGRAM · VIDEO EXPOSURE
| Lever | +1 stop of light | Side effect |
|---|---|---|
| Aperture | f/4 → f/2.8 (2× the area of the opening) | Shallower depth of field; with some lenses a softer image at the edges |
| Shutter | 1/100 → 1/50 s | More motion blur; changes the look of motion |
| ISO / EI | 400 → 800 | More visible noise; the distribution of dynamic range changes |
| ND filter | ND 0.6 → ND 0.3 | None on the picture (with a good-quality filter) |
ND filters are labelled by optical density: 0.3 = 1 stop, 0.6 = 2 stops, 0.9 = 3 stops, 1.2 = 4 stops. Some manufacturers use the notation ND2, ND4, ND8 (the filter factor), i.e. 1, 2 and 3 stops.
Aperture and T-stops
The f-number is lens geometry: the ratio of the focal length to the diameter of the entrance pupil. It doesn’t tell you how much light is lost in the glass. That is why cine lenses are marked with a T-stop, which takes the actual transmission into account. Two lenses set to T2.8 give the same exposure; two lenses set to f/2.8 may not. When you swap lenses within a scene, that is the difference between a smooth cut and jumping brightness.
What ISO does on a digital camera
With film, ISO described the sensitivity of the emulsion. The sensor in a digital camera physically has one sensitivity — the number of photons it converts into electrons. The ISO setting changes:
- analogue or digital gain applied to the signal before or after it is converted into numbers, and
- how the signal is mapped to the output curve (Rec.709 or log) — that is, how many stops of dynamic range end up above middle grey and how many below it.
Higher ISO adds no information; it just amplifies what the sensor captured — including the noise. Hence the rule: more light on the sensor is always better than more ISO.
Native and dual native ISO
Manufacturers state a native (base) ISO — the setting at which the camera achieves the best balance of dynamic range and noise, usually with the range split evenly around middle grey. When recording log, the native ISO is often also the lowest ISO available in log mode.
Above the native ISO the camera usually shifts the range: you gain headroom in the shadows at the expense of the highlights (they clip sooner). Below the native value it is the other way round. The specific values differ for every camera and are given in the manufacturer’s documentation.
Dual native ISO means the sensor has two readout circuits with different gain. Switching to the higher base value reduces shadow noise considerably compared with digitally amplifying from the lower value. In practice: if a camera has native ISOs of, say, 800 and 3200, a shot at ISO 3200 may have less noise than a shot at ISO 2000.
With log and RAW recording, people often talk about EI (exposure index) instead of ISO. It is a metadata instruction on how to interpret the recording for preview and how to distribute the range — the data from the sensor itself doesn’t necessarily change when you change the EI. Exactly how it works depends on the camera.
The order for setting exposure
- Frame rate — according to the project (25, 24…).
- Shutter speed — 180° as a starting point; in 50 Hz countries, with flickering lights in mind.
- ISO / EI — at the native value (or whichever of the two native values suits the lighting conditions).
- Aperture — for the depth of field you want, not for the light.
- ND filter — trims away the light left over after steps 1–4.
- Light — when there isn’t enough, add light before you raise the ISO.
How to measure exposure
The camera’s display lies: its brightness depends on the surroundings and the settings. The reliable tools are the ones that measure the signal:
- Waveform — shows the brightness of the picture from left to right. With Rec.709 output, skin should sit roughly in the middle and the highlights mustn’t hit the ceiling.
- False colour — recolours the picture according to brightness. The colours differ between manufacturers, so check the legend; don’t rely on colour alone, read the values too.
- Zebras — hatching above a set level; useful for guarding the highlights.
- Grey card — an 18% grey card in the shot gives a reference point for both exposure and colour correction.