Angle and speed are the same setting written two ways
Shutter angle comes from mechanical cinema cameras, where a rotating disc with a wedge cut out of it spun once per frame. A 180 degree opening meant the film was exposed for half of each frame interval. The conversion is direct: shutter time equals the angle divided by 360 times the frame interval, so at 24 frames a second, 180 degrees is 1/48 of a second, and at 60 frames a second the same 180 degrees is 1/120.
What makes the angle the more useful unit is that it holds the look constant. Motion blur is not about absolute exposure time, it is about how much of each frame interval the subject was moving during. Half of it looks the same at any frame rate. That is why a camera menu that offers both is not offering two settings — it is offering the same setting in the unit that suits how you are thinking about the shot.
The 180 degree convention itself is a habit rather than a law, and it is a habit built on projection at 24 frames a second. Shorter angles give crisper, more staccato frames, which is why action sequences sometimes sit at 45 or 90 degrees. Longer angles smear, which reads as dreamlike at 270 and as smudged at 360. None of them is wrong, but the audience has been trained on 180 for a century and departures from it read as a deliberate effect.
Slow motion is a conform, not an effect
Shoot at 120 frames a second and put the clip on a 24 frame timeline and each recorded second occupies five timeline seconds. Nothing is interpolated and nothing is thrown away; the same frames simply play at a fifth of the rate they were captured at. That is why overcranked footage is sharp in a way that software-slowed footage is not, and it is also why the frame rate has to be decided before the shot rather than after.
| Capture | Timeline | Playback | 10 s recorded becomes |
|---|---|---|---|
| 24 fps | 24 fps | 100% | 10 s |
| 48 fps | 24 fps | 50% | 20 s |
| 60 fps | 24 fps | 40% | 25 s |
| 120 fps | 24 fps | 20% | 50 s |
| 240 fps | 24 fps | 10% | 100 s |
The catch that surprises people is the light. Holding 180 degrees while going from 24 to 120 frames a second takes the shutter from 1/48 to 1/240, which is a bit over two and a third stops. Overcranked footage is dark footage unless the aperture, the ISO or the lighting makes up the difference, and high frame rate modes on many cameras also crop the sensor or drop the bit depth, which is a separate cost that no calculator can predict for your body.
There is one more subtlety in the blur. The blur was recorded at the capture rate, so when the clip is conformed to a slower timeline the apparent shutter angle drops by the same factor. A 180 degree exposure shot at 120 and played at 24 looks like a 36 degree exposure to the viewer — crisp and slightly strobed. Shooters who want slow motion to look soft compensate by opening the angle wider than 180 at capture.
The flicker check, and why 172.8 degrees exists
A light running on alternating current brightens and dims twice per mains cycle: 120 times a second on 60 Hz, 100 times a second on 50 Hz. If every frame is exposed for a whole number of those peaks, every frame collects the same amount of light and the footage is even. If the exposure catches two and a half peaks, some frames catch three and some catch two, and the result is a pulse or, on a rolling shutter, a band that crawls up the image.
Work that through for 24 frames a second in a 50 Hz country. One full light cycle is 1/50 of a second. Set the shutter to 1/50 and every frame spans exactly two peaks. Convert 1/50 to an angle at 24 fps: 360 times 24 divided by 50, which is 172.8 degrees. That is where the odd number in cine camera menus comes from — it is not a compromise on 180, it is the exact setting that makes 24 fps clean under 50 Hz lighting.
In a 60 Hz country the equivalent at 24 fps is 1/60, which is 144 degrees, or 1/120 which is 72 degrees. At 30 frames a second, 1/60 lands on exactly 180 degrees, which is one of the reasons 30 fps and 60 Hz feel so uneventful together. The calculator lists whichever of these are reachable at the frame rate you entered.
All of that assumes the fixture actually flickers at twice mains, and a great deal of modern lighting does not. Electronic drivers, phase dimmers and LED fixtures each have their own behaviour, and dimming an LED often changes its flicker frequency and depth at the same time. Where the manufacturer publishes a flicker figure, that is the authority. Where they do not, roll a few seconds at the settings you intend to use and step through the frames before the crew arrives.
What the angle does not control
It does not control rolling shutter skew. That comes from how long the sensor takes to read out a frame from top to bottom, which is a property of the sensor and the readout mode rather than of the shutter setting, and it is why a fast pan can lean vertical lines over even at a short angle. A shorter angle makes each frame crisper without making the skew smaller.
It does not control judder either. Judder is the beat between the capture rate and the display rate, most visible when 24 frame material is shown on a display running at some other rate, and it is a delivery-chain problem. And it has nothing to say about depth of field, which the aperture governs — if the shutter is locked by the look, the aperture is the thing that has to move to balance the exposure, and that changes the depth. The depth of field calculator shows what that costs.
Questions people ask
What shutter speed for 24 fps video?
The 180 degree convention gives 1/48 of a second, and where a camera does not offer 1/48 the nearest available setting of 1/50 is close enough to be indistinguishable. Under 50 Hz mains lighting 1/50 is also the flicker-free choice, which is where the 172.8 degree marking in cine menus comes from. Under 60 Hz mains the clean options at 24 fps are 1/60 and 1/120, which are 144 and 72 degrees, both slightly crisper than the convention.
What is the 180 degree shutter rule?
It is the habit of setting the exposure to half the frame interval, which came from rotating disc shutters where a half-open disc did exactly that. It produces an amount of motion blur that a century of cinema has trained audiences to read as normal. Shorter angles look sharper and more staccato, longer angles look smeared and dreamy. It is a stylistic default rather than a technical requirement, and departing from it deliberately is a legitimate choice as long as you know what it will look like.
Why does my slow motion look choppy instead of smooth?
Most often because the blur was captured at the high frame rate and then stretched. A 180 degree exposure at 120 frames a second is 1/240 of a second, which is a crisp frame; conform that to a 24 frame timeline and each frame is displayed five times longer than it was exposed for, so the eye sees a series of sharp stills rather than a flowing movement. The fix at capture is a wider shutter angle, closer to 360, which costs no light and adds the blur back. The other cause is that the footage was not overcranked at all and the editing software is duplicating frames.
Why do my lights flicker or band in video?
Because each frame is catching a different amount of light. A source on alternating current pulses twice per mains cycle, and unless the exposure covers a whole number of those pulses the frames come out unequal. On a rolling shutter the unevenness shows up as bands that drift through the frame rather than as whole-frame pulsing. Setting the shutter to a multiple of half the mains period fixes it for simple fixtures. Modern LED and dimmed fixtures flicker at their own driver frequency instead, so the settings-based fix may not work and the test has to be empirical.
Does a higher frame rate need more light?
If you hold the shutter angle, yes, and the amount is exactly the ratio of the frame rates in stops. Going from 24 to 60 frames a second at a constant 180 degrees takes the exposure from 1/48 to 1/120, which is about one and a third stops. Going to 120 costs a bit over two and a third. That has to come from somewhere: a wider aperture, which changes the depth of field, higher ISO, which changes the noise, or more light on the subject. Many cameras also crop or reduce readout quality in high frame rate modes, which is a separate cost worth checking on your own body.