180-degree shutter rule

At 24fps that means 1/48 of a second, or 1/50 on cameras offering only standard steps. At 60fps it is 1/120. The number comes from film cameras with a rotating disc shutter open across half of each rotation, and audiences have been trained on that look for a century. Go much faster and motion turns crisp and stuttery, which is why war films use it deliberately. Go slower and movement smears. Outdoors it pushes you towards neutral density filters, since the only remaining ways to control exposure are aperture and ISO.