Shutter speed and the 180° rule
In stills photography, shutter speed is mainly an exposure control. In video it decides how motion looks — which is why it is tied to the frame rate, not to the light.
What the shutter does in video
Every frame of video is a short exposure. Shutter speed tells you how long the sensor gathers light during a single frame. At 25 frames per second one frame lasts 40 ms. If the shutter is set to 1/50 s, the sensor gathers light for 20 ms and the remaining 20 ms is ‘dark’ — the camera reads out the sensor and gets ready for the next frame.
Anything that moves during those 20 ms is blurred in the frame. And it is exactly this blur that the eye joins into smooth motion during playback. A short shutter gives sharp, ‘frozen’ frames that look choppy when played back. A long shutter gives streaks and soft, even ‘dreamy’ motion.
Shutter angle: where it comes from
Film cameras had a rotary shutter — a disc with a cut-out that turns once per frame in front of the film gate. When the cut-out is in front of the gate, the film is exposed; when the solid part of the disc is in front of it, the film is pulled down by one frame. The size of the cut-out is given in degrees. A half circle, or 180°, means the film is exposed for half the time of each frame.
The advantage of an angle over a time is that it describes the look of motion independently of the frame rate. 180° at 24 fps and at 60 fps means the same share of blur relative to the frame duration.
The conversion is simple:
shutter speed = (angle ÷ 360) × (1 ÷ fps)
angle = shutter speed × fps × 360
DIAGRAM · SHARE OF EACH FRAME THAT IS EXPOSED
The 180° rule in numbers
The 180° rule says: set the shutter speed to roughly double the frame rate. It is a starting point that audiences are used to from cinema and television — not a law.
| Frame rate | Frame duration | Exactly 180° | Nearest setting on stills cameras |
|---|---|---|---|
| 23.976 / 24 fps | 41.7 ms | 1/48 s | 1/50 s (= 172.8° at 24 fps) |
| 25 fps | 40 ms | 1/50 s | 1/50 s |
| 29.97 / 30 fps | 33.3 ms | 1/60 s | 1/60 s |
| 50 fps | 20 ms | 1/100 s | 1/100 s |
| 59.94 / 60 fps | 16.7 ms | 1/120 s | 1/120 s or 1/125 s |
| 120 fps | 8.3 ms | 1/240 s | 1/240 s or 1/250 s |
The difference between 1/48 and 1/50 s at 24 fps is 7.2° — impossible to spot in the picture. It does matter in practice, though, with lights powered from the mains; more on that below.
Try it yourself
Set the angle and the frame rate. The calculator shows the exact time, and below you will see a single frame of an object that crosses the picture in one second: the length of the streak is speed × exposure time.
INTERACTIVE · SHUTTER ANGLE
Shutter angle and motion blur
open — light passes closed — film advances / sensor is read out
exposure time = (angle ÷ 360) × (1 ÷ fps)
When to break the rule
A narrower angle (e.g. 45°–90°). Each frame is sharper and motion looks staccato and nervous. It is used deliberately in action and fight scenes, for raindrops, sparks and dust that should read as points, and when you need sharp stills from the video (for motion analysis or VFX tracking, for example). The cost: half the angle = one stop less light.
A wider angle (270°–360°). More blur, softer motion, slightly more light. It helps in low light when you don’t want to push the ISO any further. With fast pans, though, the streaks blur the background too and the picture can look out of focus.
High frame rates for slow motion. If you shoot 120 fps for slow motion, 180° means 1/240 s. Once slowed down to 24 fps, each frame carries blur corresponding to 1/240 s — so the slow-motion movement looks crisp, which is usually what you want in a slow-motion shot.
Shutter blur can’t be reliably removed in post-production. It can be added (a motion blur effect), but that is an estimate of the motion. If you don’t know where a shot will end up, shoot with a narrower angle — you can add blur, but you can’t ‘un-blur’.
Flickering lights at 50 Hz
Fluorescent tubes, many LED fixtures and discharge lamps running on AC mains change brightness in time with the mains. In Slovakia the mains run at 50 Hz, so the brightness of these lights fluctuates 100 times a second. If the shutter speed doesn’t cover a whole number of these cycles, each frame catches a different part of the cycle and the picture flickers or shows bands (with a rolling shutter).
- Safe shutter speeds at 50 Hz: 1/50 s, 1/100 s (whole multiples of 1/100 s). That is why in Europe 1/50 s, i.e. 172.8°, is often chosen at 24 fps rather than 1/48 s.
- With 60 Hz mains (USA, part of Japan) lights flicker 120 times a second and 1/60 and 1/120 s are safe. At 25 fps in a 60 Hz country that means, for example, 150° (1/60 s).
- Good-quality film LED fixtures have flicker-free power supplies or an adjustable PWM frequency. Test them before the shoot anyway — ideally at the planned angle and frame rate, and in slow motion too.
Rolling shutter: the second ‘shutter’
Most CMOS sensors don’t read the whole image at once but row by row from top to bottom. If the readout is slow, fast camera or subject movement skews vertical lines (‘jello’, leaning pillars), helicopter rotors and lightning look strange, and photographers’ flashes split the frame into a bright half and a dark half. This effect has nothing to do with the shutter angle you set — it depends on how fast the sensor is read out. Cameras with a global shutter expose the whole sensor at once and don’t have this problem.