Cinema Shutter Angle & 180° Rule vs. T-Stop
Comparing 2 standards across 10 specifications. 10 key differences identified.
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.
The T-Stop (Transmission Stop, standardized under ISO 517 for cinematography) is a photometric measurement of a lens's true light transmission (T = f / √Transmittance), correcting the theoretical geometric f-stop for physical light lost to glass reflection and optical element absorption. Standardized across all cine primes (Cooke, ARRI, Zeiss, Angénieux).
Specification Comparison Table
10 Parameters| Parameter / Spec | ||
|---|---|---|
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Mathematical Formula
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Shutter Speed (t) = 1 / (Frame Rate × (360° / Shutter Angle)) | T-Number = F-Number / √(Transmittance Percentage as a decimal) |
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180° Standard Shutter Rule
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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 | — |
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90° Narrow Shutter (Action Effect)
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Fast exposure (1/96 sec at 24 fps) — crisp, staccato, jittery combat motion (Saving Private Ryan effect) | — |
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360° Open Shutter (Dream / Smear)
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Full-duration exposure (1/24 sec at 24 fps) — extreme motion blur, dreamy light trails, maximum low-light intake | — |
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172.8° Shutter Angle (Flicker Reduction)
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Eliminates 50 Hz European AC electrical lighting flicker when shooting at 24 fps in PAL regions | — |
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Cinema Camera Standard
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ARRI Alexa, RED, Sony FX6/FX9, Canon C300, and Blackmagic display shutter in degrees to maintain constant motion blur during frame rate ramps | — |
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Physical Meaning
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— | A lens set to T2.8 transmits the exact same photometric illumination to the sensor as an ideal 100% transparent lens at f/2.8 |
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Complex Zoom Glass Loss
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— | A 18-element zoom marked f/2.8 with 80% transmission has an actual transmission rating of T3.1 (0.3 stops darker than f/2.8) |
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Cinema Iris Ring Markings
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— | Marked strictly in T-stops with linear mechanical spacing and clickless 0.8 MOD gears for follow focus rigs |
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Shot-to-Shot Matching
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— | Switching from a 24mm wide lens to a 85mm telephoto on a movie set, both set to T2.8, is designed to hold exposure and luminance effectively constant across the cut |