Frame rates and shutter: the 180 degree rule and flicker-free shutters for UK 50Hz
The 180 degree rule gives natural motion blur. In the UK, the shutter speed also has to suit 50Hz mains lighting, or the picture will flicker or band.
In short: A 180 degree shutter is 1 divided by twice the frame rate: 1/50 of a second at 25 frames per second, 1/48 at 24. Under UK mains-powered lights, 1/50 or 1/100 of a second avoids flicker. At 25 fps, 1/50 gives 180 degrees and a clean picture. At 24 fps, 1/50 is about 173 degrees, the usual compromise.
What the rule is for
The 180 degree shutter gives each frame the motion blur the eye expects from film. A shutter open for half of each frame time gives that blur. Changing the shutter angle changes how sharp moving subjects look, so it is one of the first settings to fix on a shoot.
The formula
shutter angle = 360 × frame rate × shutter time
Rearranged for a 180 degree shutter, the shutter time is half the frame time:
shutter time = 1 ÷ (2 × frame rate)
| Frame rate | 180 degree shutter | Flicker-free in UK 50Hz? |
|---|---|---|
| 24 fps | 1/48 s | No, use 1/50 s (about 173 degrees) |
| 25 fps | 1/50 s | Yes |
| 30 fps | 1/60 s | No. Shoot at 25 or 50 fps for UK light |
| 50 fps | 1/100 s | Yes |
Why UK lighting cares
Mains in the UK runs at 50Hz. Many lamps and LEDs flicker at twice the mains frequency, 100 times a second. A shutter that is a whole multiple of 1/100 of a second averages the flicker out. Shutter speeds such as 1/60 are not multiples of 1/100, so the picture bands as the lights and the shutter drift in and out of step.
The rule is about the mains, not the frame rate. A 60Hz habit carried over from US shoots is a common cause of banding on UK shoots, because 1/60 looks correct on the camera's dial and is wrong for UK light.
Some LEDs pulse for their own reasons, such as dimming by pulse width, so they may flicker whatever the shutter. Test each fixture on the camera before you commit.
A worked example
You are shooting an interview at 25 frames per second in a room lit by LED panels:
- The 180 degree shutter is 1/50 of a second.
- 1/50 is a multiple of 1/100, so the mains flicker is averaged out.
- The shutter stays at 1/50 while you shoot at 25 frames per second. If you change to 50 frames per second for a slow-motion insert, use 1/100 to keep 180 degrees.
If the shoot is at 23.976 frames per second for a film look, 1/48 is the 180 degree figure, but it flickers under UK light. Use 1/50 instead, which is about 173 degrees. The motion is slightly sharper and the picture stays clean.
Common mistakes
- Setting 1/60 from habit. It looks right on the dial and bands on UK lighting.
- Changing the frame rate and forgetting the shutter. A 25 to 50 frames per second change needs the shutter to move from 1/50 to 1/100.
- Using a fast shutter for a sharper look without thinking about the blur. Fast shutters give a stuttering, sharp motion that can be wrong for drama.
- Assuming a flicker-free shutter fixes every fixture. Some LED dimmers cause flicker of their own.
Where it fits
The shutter and frame rate set the motion, and the storage follows from the frame rate and bitrate. Use the data rate and storage calculator after you have chosen the settings, and the field of view calculator for the frame.
Sources
- Alister Chapman, The obsession with using shutter angle on an electronic video camera, read 5 November 2026
Related guides
- Field of view and focal length calculator, by sensor
- Depth of field calculator, with a worked example
- Data rate and storage calculator for camera files
- Lens and sensor crop cheat sheet
- Camera report template: what to log on each take
- All guides and tools
The picture at the top of this page is an illustration, not a photograph of a client or a real job.
Questions we get asked
What shutter speed should I use at 25 frames per second in the UK?
1/50 of a second. That gives a 180 degree shutter and avoids flicker under 50Hz mains lighting.
Why does my footage flicker or band under LED lights?
Either the shutter is not a whole multiple of the lighting's flicker rate, or the light pulses for its own reasons. In the UK, use 1/50 or 1/100 of a second, and test LED fixtures on the camera before the shoot.
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