Exposure for video
In photography you can play freely with shutter speed. In video you can't: it's tied to the frame rate. That's why exposure works a little differently in video and needs a tool you hardly ever use in photography: the ND filter.
The 180-degree rule
The most famous rule in video says: your shutter speed should be double your frame rate, or as close to it as possible.
| You shoot at | Shutter by the rule | In practice |
|---|---|---|
| 24 fps | 1/48 | 1/50 |
| 25 fps | 1/50 | 1/50 |
| 30 fps | 1/60 | 1/60 |
| 50 fps | 1/100 | 1/100 |
| 60 fps | 1/120 | 1/125 |
Why? Because at that speed each frame has just the right amount of motion blur for movement to look natural. It's the look cinema has taught us to expect. The name comes from film cameras, which had a half-moon-shaped rotary shutter: open for half of each turn, 180 out of 360 degrees.
If you use a much faster speed (1/500 at 25 fps), every frame comes out sharp and movement looks choppy and jerky. If you use a slower one (1/25), everything smears. Try it with the cyclist in the simulator above.
War and action scenes are often shot with a very fast shutter to create a feeling of chaos and extreme sharpness. Dreamy music videos use 360°. The rule exists so you can break it knowing what you'll get.
The problem: too much light
Picture a sunny day. You're shooting at 25 fps, so your shutter is locked at 1/50 (or 30 fps at 1/60). You want a blurry background, so you open the aperture to f/2. You drop the ISO to the minimum… and the image is still blown out. In photography you'd raise the shutter speed to 1/4000 and be done. In video you can't without ruining the motion.
That leaves you closing the aperture to f/16, losing the background blur and the look you were after. Or using an ND filter.
ND filters: sunglasses for your camera
An ND (neutral density) filter is a piece of gray glass that reduces the light coming in without changing the colors. It lets you keep 1/50 and f/2 in full sun.
| Filter | Stops of light it cuts | Typical use |
|---|---|---|
| ND2 | 1 | Bright interior |
| ND4 | 2 | Overcast day |
| ND8 | 3 | Shade on a sunny day |
| ND16 | 4 | Sunny exterior at a medium aperture |
| ND64 | 6 | Full sun with a very wide aperture |
| ND1000 | 10 | Long exposure photography; almost never in video |
There are two types:
- Fixed ND: better optical quality, but you have to swap it depending on the light.
- Variable ND: two polarizers you rotate to dial in roughly 2 to 8 stops. Very handy for beginners. Cheap ones create a dark cross in the image at full strength, the so-called "X"; don't turn the ring all the way to the stop.
Phones have a fixed aperture and adjust for light using shutter speed. That's why phone video in full sun looks very sharp, like a video game. There are clip-on ND filters for phones, and apps with manual controls that let you lock the shutter at 1/50 (or 1/60).
How many stops of ND do you need? Work it out with the exposure and ND calculator.
The order for exposing video
- Fps: choose the frame rate (24 or 30 in the US, 25 in the UK and Europe).
- Shutter: double the fps (1/50 or 1/60).
- Aperture: based on the depth of field you want.
- ISO: your camera's native or base ISO. Raise it only if you're short of light.
- ND: if there's too much light, add filtration until the exposure is right.
Tools for measuring exposure
Don't trust the camera screen: its brightness changes with the ambient light. Use these aids:
- Zebras: diagonal stripes over areas that go above a set level. Set them to 95–100% to warn you about what's blowing out, or to 70% for mid-tone skin.
- Histogram: the same one as in photography. Keep it from piling up against the right edge.
- False color: paints the image in colors according to brightness. It's the professionals' tool.
With auto exposure, if someone in a white shirt walks into the shot, the camera suddenly darkens the whole image. In video those changes are visible. Set exposure manually or lock it before you start recording.
Native ISO and noise
Many video cameras have one or two native ISOs: values at which the sensor delivers its best dynamic range. In log profiles it's usually high (800 or 640, for example). Check your camera's manual. An ISO below native doesn't always reduce noise; sometimes it only reduces dynamic range.
- Shutter = double the fps (1/50 at 25 fps, 1/60 at 30 fps).
- In video you control light with aperture, ISO and ND filters.
- Zebras and the histogram beat the screen.
- Never leave exposure on auto if someone or something changes in the shot.
Your turn: the three-shutter walk
Film someone walking past the camera or waving their hand, at 25 fps (or 30) with three different shutter speeds.
- One at 1/50 (or 1/60 at 30 fps), following the rule.
- Another at 1/500 or faster. Compensate with aperture or ISO.
- Another at 1/25 (or 1/30). Watch the three back to back and describe in one sentence what each one conveys.
Frequently asked questions
What happens if I don't follow the 180-degree rule?
The video still records, but the movement changes: with a very fast shutter it looks choppy, and with a slow shutter it smears. You can break it on purpose as a stylistic choice.
Which ND filter should I buy to start?
A good-quality variable ND of about 2 to 5 stops covers most situations. Buy it in the diameter of your largest lens and use step-up rings for the others.