How Long Exposures Erase People: The Math of Making Crowds Vanish

Put a 35mm lens on a full frame camera, stand about 20 m back from a plaza, and each pixel in the file covers roughly 3.4 mm of the ground at that distance. A person walking across that view at an ordinary pace clears their own shoulder width in about a third of a second, so across a four-minute exposure they sit over any single pixel for about 0.13 percent of the time. That fraction is the whole mechanism, and once you can compute it, and then combine the occupancy of everyone walking the same