Shutter Angle & 180° Rule Calculator

How it works

Shutter speed = 1 ÷ (fps × 360 ÷ angle); 180° → 1/(2 × fps); flicker-safe: exposure = n ÷ (2 × mains Hz)

Film cameras expose each frame while a rotating disc with a cut-out sweeps past the gate; a 180-degree opening exposes for exactly half of each frame’s duration, so at 24 fps the shutter is open for 1/48 second. That amount of motion blur is what audiences have learned to read as “cinematic”, hence the 180-degree rule: set the shutter speed to double the frame rate. Smaller angles freeze motion and look jittery (used deliberately in action sequences); larger angles smear it. Digital cameras express the same thing as a shutter speed, and when the frame rate changes — slow-motion at 120 fps, say — the speed must change with it to keep the look. Artificial light pulses at twice the mains frequency, so exposures that are not whole multiples of 1/100 or 1/120 s can produce rolling flicker.

Worked example

24 fps at 180°: 1/48 s (20.8 ms). Shooting 120 fps slow motion at the same angle needs 1/240 s. Under 50 Hz lights, 1/50 or 1/100 avoids flicker.

Frequently asked questions