Video Shutter Speed & 180° Rule Calculator
Calculate cinema reciprocal shutter speeds, exposure time in milliseconds, and required ND filter density for 180° and custom shutter angles at any framerate.
Cinematography Framerate & Shutter Angle
Understanding the 180° Shutter Rule and ND Filtration
In traditional rotary disc cinema cameras, a spinning half-circle shutter disc exposed film for half of each frame cycle, establishing the 180-degree shutter rule. This produces natural motion blur that matches human optical persistence.
Mathematical Exposure Formulas
- Shutter Speed Denominator:
Denominator = (360° × Framerate) / Shutter Angle
For standard 180° cinema at 24 fps:(360 × 24) / 180 = 48 → 1/48 second. - Exposure Time (milliseconds):
Exposure (ms) = 1000 / Denominator = (Shutter Angle × 1000) / (360 × Framerate)
For 24 fps at 180°:1000 / 48 = 20.83 ms. - ND Filter Exposure Reduction:
Stops Required = log₂ (Ambient Daylight Shutter / Target Cinema Shutter)
If outdoor sunlight requires 1/1000s at f/2.8 and your cinema target is 1/50s:log₂(1000 / 50) = log₂(20) = 4.32 stops → ND16 (4 stops) to ND32 (5 stops).
Frequently Asked Questions
Why do mirrorless cameras show 1/50s instead of 1/48s for 24fps?
Still cameras and hybrid mirrorless bodies typically increment shutter speeds in standard photographic 1/3-stop increments (1/30, 1/40, 1/50, 1/60). 1/50s is within 4% of 1/48s and produces indistinguishable cinematic motion blur.
When should I use a 90-degree or 45-degree shutter angle?
Narrow shutter angles (fast shutter speeds such as 1/96s or 1/192s at 24fps) dramatically reduce motion blur, creating a crisp, intense, staccato look. This was famously used in Saving Private Ryan and Gladiator for battle sequences.
What is the difference between optical density (OD) and ND filter numbers?
Different filter manufacturers use different labeling: ND 0.3 = 1 stop (ND2); ND 0.6 = 2 stops (ND4); ND 0.9 = 3 stops (ND8); ND 1.2 = 4 stops (ND16); ND 1.5 = 5 stops (ND32); ND 1.8 = 6 stops (ND64); ND 3.0 = 10 stops (ND1000).