Vivian Maier looked after other people’s children in Chicago for about forty years and took well over a hundred thousand photographs: strangers on the pavement, kids on her rounds, her own shadow stretched across a sidewalk. She showed almost nobody. When her negatives turned up at an auction in 2007, a great many of the rolls had never been developed. She had pressed the shutter and never found out how the picture came out. We assume the point of a photograph is the print. For her it may have been the half-second of looking, and the film was only a receipt for having paid attention.