Shutter angle calculator

Cinema cameras describe the shutter in degrees. Most mirrorless cameras and phones describe it in fractions of a second. Both mean the same thing, how long each frame is exposed, and a crew that mixes the two needs a quick way to translate.

The degrees come from film cameras, where a spinning disc sat in front of the gate with part of it cut away. While the open wedge passed the film, light got through. A disc open across half its circle, 180 degrees, exposed each frame for half the time that frame occupied. Digital cameras have no disc. The number still describes the same proportion.

How the shutter angle calculator works

Shutter speed is the exposure time of one frame. The calculator ties it to the angle with a single formula:

exposure time in seconds = angle ÷ (360 × frames per second)

Cameras show that as 1/x, where x = 360 × frames per second ÷ angle. Run it backwards and angle = 360 × frames per second ÷ x. Choose "shutter angle" or "shutter speed (1/x)" in the "I know the" menu and type whichever number you have.

Two more lines follow. The closest common camera setting picks the nearest speed from a list of standard values between 1/24 and 1/1000 s, and gives the angle that speed produces at your frame rate. The flicker line checks the exposure against your mains power. Lights on alternating current pulse twice per cycle: 100 times a second on 50 Hz power, 120 times on 60 Hz. An exposure that lasts a whole number of pulses collects the same light every frame. One that lasts two and a half pulses does not, and the picture throbs. When that happens the tool suggests 1/(2 × mains), 1/mains or 1/(2 × mains ÷ 3), which on 60 Hz power means 1/120, 1/60 or 1/40 s.

The 180 degree shutter rule

Set the shutter to half the frame interval. At 24 fps that is 1/48 s, and it gives the motion blur audiences have absorbed from a century of cinema: each frame smeared just enough that movement reads as smooth.

Breaking the rule changes the look. Narrow angles of 90 or 45 degrees freeze each frame, so fast motion turns crisp and staccato, striking for rain or sparks and tiring over a long scene. Wide angles of 270 or 360 degrees smear movement and soften action. They also let in more light, which is why low-light documentary work sometimes goes there on purpose. Every halving of the angle costs one stop.

In bright sun the temptation is to raise the shutter speed to tame exposure. A neutral density filter does that job without changing the motion. One more thing: this rule has nothing to do with the 180 degree rule of screen direction, which concerns where the camera stands.

Worked example: 24 fps at 180 degrees in North America

The tool opens at 24 fps, shutter angle mode, 180 degrees and 60 Hz mains. It reads:

• Shutter speed: 1/48 s
• Shutter angle: 180°
• Closest common camera setting: 1/48 s, which is 180° at 24 fps
• Flicker under 60 Hz lights: may flicker, try 1/120, 1/60 or 1/40 s

That last line matters most. 1/48 s lasts 20.8 milliseconds. A light on 60 Hz power pulses every 1/120 s, about 8.3 milliseconds, so each exposure catches two and a half pulses. Switch the mode to shutter speed and type 60: at 24 fps, 1/60 s is 144 degrees, slightly crisper than 180, and it spans exactly two pulses. Under the fluorescent tubes of a school or an office, that is the setting to reach for.

Type 50 instead and the tool gives 172.8 degrees. That speed spans two pulses of 50 Hz light, so it is flicker-safe in Europe and much of the world, which is why 24 fps shooters there use it. On 60 Hz power it is not safe.

Other frame rates follow the same rule. At 25 fps, 180 degrees is 1/50 s, safe under 50 Hz lights. At 29.97 fps it is 1/59.94 s, and the tool offers 1/60 as the nearest camera setting.

Shutter speed chart at 24 fps

The chart under the tool redraws for whichever frame rate you choose. At 24 fps:

Read the look column as a rough guide. At 144 and 172.8 degrees the difference from 180 is slight, and few viewers would spot it.

What the calculator cannot tell you

It cannot tell whether a particular light will flicker. A tungsten filament stays hot between pulses and barely changes. Fluorescent tubes, sodium street lamps and many LEDs do change, and some LED fixtures dimmed electronically flicker at rates unrelated to the mains. Before an interview, record ten seconds at your settings and play it back on a decent monitor; the interview lighting guide has more on choosing and taming the light in a room.

It also says nothing about screens in shot, which refresh at their own rates, or about rolling-shutter skew on fast pans. Exposures shorter than one light pulse, faster than 1/120 s on 60 Hz power, are flagged as possible flicker because some fixtures show it at those speeds and some do not. And if you are matching frame rates across cameras, the timecode calculator explains why 29.97 is not quite 30.

Questions

What is the 180 degree shutter rule?

Set the shutter speed to one over twice the frame rate: 1/48 s at 24 fps, 1/50 s at 25 fps, 1/60 s at 30 fps. It produces the motion blur most viewers read as natural.

What shutter speed should I use at 24 fps?

1/48 s if the camera offers it, otherwise 1/50 s, which is 172.8 degrees. Under 60 Hz mains lighting in North America, use 1/60 s (144 degrees) to avoid flicker.

Why does my footage flicker under indoor lights?

Mains-powered lights pulse 120 times a second on 60 Hz power and 100 times on 50 Hz. If the exposure does not span a whole number of pulses, each frame catches a different amount of light. Use 1/60 or 1/120 s on 60 Hz power, or 1/50 or 1/100 s on 50 Hz.

What is shutter speed in video?

The time each frame is exposed, written as a fraction of a second. In normal recording it cannot be longer than one frame: at 24 fps the slowest is 1/24 s, a 360 degree shutter, which is where the calculator stops.