What this calculator does
Film cameras exposed each frame through a rotating disc with a wedge cut out of it, and the size of that wedge was the shutter angle. A 180 degree shutter meant the disc was open for half of each frame, which is where the convention of shooting at double your frame rate comes from.
Digital cameras have no disc, but the convention survived because the motion blur it produces is what audiences read as film. At 24 frames a second a 180 degree shutter is 1/48, and the familiar advice to use 1/50 is a small approximation that most cameras force on you because 1/48 is not on the dial.
The formula
The shutter is open for the fraction angle ÷ 360 of each frame, and each frame lasts 1 ÷ frame rate seconds, so the exposure time is the two multiplied. Rearranging for the angle that gives a particular shutter speed gives angle = 360 × frame rate ÷ denominator, which is how to match a 1/50 shutter at an unusual frame rate. An angle cannot exceed 360 degrees, so above a certain frame rate a slow shutter is simply unreachable.
| Term | Meaning |
|---|---|
| Shutter angle | The fraction of each frame the shutter is open, expressed as degrees of a circle. |
| 180 degree rule | Shutter open for half of each frame. The convention for natural looking motion blur. |
| Frame rate | Frames captured per second. Changes the shutter speed for a given angle. |
| Stops | A doubling of the angle is one stop more light, exactly as with any other exposure control. |
The inputs explained
| Field | What to enter |
|---|---|
| Frame rate (fps) | The frame rate you are shooting at, which for slow motion is the capture rate rather than the playback rate. |
| Shutter angle (°) | Shutter angle in degrees. 180 is the default convention, 90 gives a sharper and more staccato look, 360 is the maximum. |
| Shutter speed you want to match (1/x) | The denominator of a shutter speed you would rather dial in, used to work out which angle matches it. Enter 50 for 1/50. |
When to use it
Keeping the look consistent across frame rates
Shooting at 24 and 60 in the same production, a fixed 180 degree shutter keeps the motion blur consistent even though the shutter speeds are 1/48 and 1/120. Fixing the shutter speed instead would make the two look different.
Matching a camera that only offers shutter speeds
If your camera has no angle setting, use the last output. At 24 fps a 1/50 shutter is 172.8 degrees, close enough to 180 that nobody will see the difference.
Deliberately sharpening the motion
Drop the angle to 90 and the shutter speed halves, giving the crisp stuttering motion used in action sequences. It also costs a stop of light, which the calculator reports.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What shutter speed does a 180 degree shutter give?
The angle is fixed and only the frame rate changes.
| Frame rate | Shutter speed at this angle | Exposure time | Angle that gives your wanted speed |
|---|---|---|---|
| 24 fps | 1/48 s | 0.0208 s | 172.8° |
| 25 fps | 1/50 s | 0.0200 s | 180.0° |
| 30 fps | 1/60 s | 0.0167 s | 216.0° |
| 48 fps | 1/96 s | 0.0104 s | 345.6° |
| 60 fps | 1/120 s | 0.0083 s | over 360°, not reachable at this frame rate |
| 120 fps | 1/240 s | 0.0042 s | over 360°, not reachable at this frame rate |
What does changing the angle cost in light?
The frame rate is fixed and the angle narrows.
| Shutter angle | Shutter speed at this angle | Light against a 180 degree shutter | Share of each frame the shutter is open |
|---|---|---|---|
| 360° | 1/24 s | 1.00 stops | 100.0% |
| 180° | 1/48 s | 0.00 stops | 50.0% |
| 90° | 1/96 s | -1.00 stops | 25.0% |
| 45° | 1/192 s | -2.00 stops | 12.5% |
| 22.5° | 1/384 s | -3.00 stops | 6.25% |
Questions
Why 180 degrees rather than anything else?
Convention, inherited from mechanical film cameras where a half-open disc was a practical compromise between motion blur and light. Audiences learned that look, so departures from it read as a deliberate effect.
Is 1/50 the same as a 180 degree shutter at 24 fps?
Almost. A true 180 degree shutter at 24 fps is 1/48, and most cameras offer 1/50 instead. The difference is a sixteenth of a stop and is invisible, but 172.8 degrees is the honest figure if your camera takes angles.
What happens above 360 degrees?
Nothing, because the shutter cannot be open for longer than the frame lasts. If the calculator says an angle above 360 would be needed, that shutter speed is simply unreachable at that frame rate and you must slow the frame rate instead.
Does shutter angle matter for slow motion?
Yes, and use the capture rate rather than the playback rate. Shooting 120 fps for playback at 24, a 180 degree shutter is 1/240, which is what governs the blur in the slowed footage.
How does this interact with flicker?
Mains-powered lighting flickers at twice the supply frequency, so shutter speeds that are not a multiple of that can band. It is the main reason to shoot 25 fps where the mains is 50 Hz, and the shutter speed rather than the angle is what matters.
For the stills side of the same exposure trade, see exposure equivalent, and for cutting light to allow a wider angle in daylight, ND filter exposure. Footage size is on video file size.