Your meter wants one shutter speed; the 180° rule demands another. The gap is filtration.
The arithmetic
| Quantity | Formula |
|---|---|
| Shutter time | angle ÷ (360 × frame rate) |
| Shutter angle | 360 × frame rate × shutter time |
| Stops from 180° | log₂(angle ÷ 180) |
The name is literal. A film camera's shutter was a spinning disc with a wedge cut out of it; a 180° shutter had half the disc removed, so each frame was exposed for half the time it spent in the gate. Digital sensors have no disc, but the language stuck because it survives a frame rate change — 180° means the same thing at 24 fps and at 120 fps, while "1/48" does not.
Every frame rate at 180°
| Frame rate | 180° shutter | Nearest camera setting | 50 Hz safe | 60 Hz safe |
|---|
When to break the rule
Narrow — 90° and below
Short exposure, minimal blur, hard-edged motion that stutters. Saving Private Ryan made this a genre. It also helps when you plan to stabilise in post, because sharp frames track better, and when shooting screens or fast text you need to stay legible.
Wide — 270° and above
Long exposure, smeared motion, a dreamy or disoriented feel. 360° means the sensor is open for the entire frame duration, which is as much blur as the format can produce. It also buys a full stop of light over 180°, which matters more than the look when you are out of options at night.
172.8° — the flicker dodge
At 24 fps a 172.8° shutter gives exactly 1/50 second, which is a whole number of 50 Hz mains cycles. In Europe and anywhere else on 50 Hz it removes banding under artificial light at the cost of a difference in motion blur nobody has ever seen.
Consistency beats correctness. A scene cut together from shots at 180°, 90° and 270° will feel wrong even to a viewer who could not name why. Pick an angle for the scene, then hold it.
Common questions
What is the 180 degree rule?
Set the shutter speed to about double the frame rate — 1/48 at 24 fps, 1/60 at 30 fps. It reproduces the motion blur of a film camera's half-open rotating shutter, which is the blur audiences have been trained to read as normal.
Why 1/50 instead of 1/48 at 24 fps?
Because most cameras do not offer 1/48, and because 1/50 is flicker-safe on 50 Hz mains. The difference is 172.8° versus 180°, which is under a tenth of a stop and invisible.
Does shutter angle affect exposure?
Directly. Halving the angle halves the light, exactly like a stop. That is why changing your angle to fix exposure is a trap — you fix the brightness and change the motion rendering at the same time.
What shutter speed for 60 fps slow motion?
1/120 second for 180°. Note that shooting 60 fps for playback at 24 also costs you 1.3 stops relative to shooting 24 fps at 180°, because the frames are shorter. Slow motion is expensive in light.
Can an iPhone set a shutter angle?
Not natively — the stock camera app decides. A manual camera app can set the shutter speed directly, which is the same thing once you know the frame rate. Lucid Camera pins a 180° shutter automatically in video mode and floats ISO instead.
Related tools
A 180° shutter, held for you.
Lucid's video mode pins the shutter to your frame rate and floats ISO through the same exposure engine as stills — with live LUT preview and audio meters.