SUMMARY
Making crowds vanish in long exposure photography relies on calculating how long a person occupies a sensor pixel relative to the total exposure time. A person walking across a frame for a fraction of a second contributes minimally to a pixel's total light count, often below the noise threshold. However, individuals who stop or remain stationary for extended periods can become visible, appearing as translucent figures. The total occupancy of a pixel by multiple people, rather than individual crossings, is key, with overlapping movements potentially canceling each other out.
TAKEAWAYS
Dwell time and contribution fraction determine if people disappear in long exposures.
A person's movement speed and width dictate how long they cover a pixel.
Stationary subjects or prolonged stops make individuals visible in long exposures.
Total pixel occupancy by multiple people, not just individual crossings, is crucial.