Cinema Shutter Angle & 180° Rule
Direct Answer
Shutter Angle (measured in degrees from 0° to 360°, originating from circular rotary disc shutters in 35mm motion picture cameras) is the cinematic standard for measuring exposure time relative to frame rate (Shutter Speed = Frame Rate × 360° / Shutter Angle). The universal '180-Degree Shutter Rule' (exposure time = 1 / (2 × Frame Rate)) delivers natural, organic motion blur that mimics human eye persistence.
Source: SMPTE (Society of Motion Picture and Television Engineers) / ASC Cinematography Manual
Specs
| Mathematical Formula | Shutter Speed (t) = 1 / (Frame Rate × (360° / Shutter Angle)) |
| 180° Standard Shutter Rule | Exposure time is exactly half of the frame duration: at 24 fps = 1/48 sec | at 30 fps = 1/60 sec | at 60 fps = 1/120 sec | at 120 fps = 1/240 sec |
| 90° Narrow Shutter (Action Effect) | Fast exposure (1/96 sec at 24 fps) — crisp, staccato, jittery combat motion (Saving Private Ryan effect) |
| 360° Open Shutter (Dream / Smear) | Full-duration exposure (1/24 sec at 24 fps) — extreme motion blur, dreamy light trails, maximum low-light intake |
| 172.8° Shutter Angle (Flicker Reduction) | Eliminates 50 Hz European AC electrical lighting flicker when shooting at 24 fps in PAL regions |
| Cinema Camera Standard | ARRI Alexa, RED, Sony FX6/FX9, Canon C300, and Blackmagic display shutter in degrees to maintain constant motion blur during frame rate ramps |
Rotary Disc Shutter Mechanics
In traditional 35mm film movie cameras, a half-circle mirror disc ($180^\circ$) rotated at the film frame rate. While the open half exposed the film for $1/48\text{ s}$, the solid mirrored half reflected light into the optical viewfinder while a mechanical claw pulled down the next film frame. In digital cinematography, setting shutter angle to 180° automatically adjusts shutter speed whenever changing slow-motion frame rates, preserving consistent cinematic motion blur without manual recalculation.