F-Stop Scale vs. Cinema Shutter Angle & 180° Rule
Comparing 2 standards across 12 specifications. 12 key differences identified.
The F-Stop Scale is a standardized geometric progression of lens focal ratios (N = f / D, where f is focal length and D is entrance pupil diameter) governed by powers of √2 ≈ 1.4142. Advancing one full stop (e.g. from f/2.8 to f/4.0) halves the cross-sectional area of the optical entrance pupil, cutting light transmission precisely in half (-1 EV).
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.
Specification Comparison Table
12 Parameters| Parameter / Spec | ||
|---|---|---|
|
Mathematical Definition
|
F-Number N = Focal Length (f) / Entrance Pupil Diameter (D) | — |
|
Geometric Progression Factor
|
Powers of √2 (~1.41421356...): √2⁰=1.0, √2¹=1.4, √2²=2.0, √2³=2.8, √2⁴=4.0, √2⁵=5.6, √2⁶=8.0, √2⁷=11.3, √2⁸=16.0, √2⁹=22.6 | — |
|
Standard Full-Stop Scale
|
f/1.0 -> f/1.4 -> f/2.0 -> f/2.8 -> f/4.0 -> f/5.6 -> f/8.0 -> f/11 -> f/16 -> f/22 -> f/32 | — |
|
Standard 1/3-Stop Increments
|
f/1.4, f/1.6, f/1.8, f/2.0, f/2.2, f/2.5, f/2.8, f/3.2, f/3.5, f/4.0, f/4.5, f/5.0, f/5.6, f/6.3, f/7.1, f/8.0, f/9.0, f/10, f/11, f/13, f/14, f/16 | — |
|
Light Transmission Rule
|
Each full f-stop increase cuts light passing through the lens by exactly 50% (1 stop / 1 EV) | — |
|
Diffraction Limit Physics
|
Airy disk diameter = 2.44 × λ × N. At f/16 and f/22 on high-res digital sensors, diffraction visibly softens fine detail | — |
|
Mathematical Formula
|
— | Shutter Speed (t) = 1 / (Frame Rate × (360° / Shutter Angle)) |
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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 |
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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 |