Photography has no single inventor. From the 1820s Joseph Nicéphore Niépce produced the first permanent camera image on a bitumen-coated pewter plate; after Niépce's death in 1833 his partner Louis Daguerre developed a silver-coated copper plate, fumed with mercury vapour, that yielded astonishingly detailed but unique images. In the same years, in Wiltshire, Henry Fox Talbot was working out a reproducible negative-positive process using silver salts on paper. Unknown to each other, the two men reached almost the same threshold.

1839 was the year that threshold became public. In January, Talbot presented "photogenic drawing" to the Royal Society. The French government bought Daguerre's process and, on 19 August, presented it as "a gift to the world" at a joint session of the Academies of Sciences and Fine Arts — the patent was released free of charge, except in Britain. Daguerre's 1838 long-exposure view of the Boulevard du Temple, in which a man pausing to have his boots polished became the first human ever photographed, was already a year old.

The consequences arrived with dizzying speed. By the 1840s portrait studios were opening in Paris, London, Istanbul and New York; the painted likeness, until then the privilege of aristocrats, came within reach of the middle class. In 1855 Roger Fenton brought back photographs from the Crimean War — the visual record of war no longer depended on a painter's interpretation. Scientists turned the camera on the Moon, the cell, the galloping horse. In 1888 George Eastman's Kodak — "You press the button, we do the rest" — placed the camera in ordinary hands.

Photography also permanently changed the status of painting. Likeness was no longer the painter's task; the road from Impressionism to abstract art is in part a consequence of that handover. More deeply, photography altered the human relationship to memory: a face, a moment, a landscape were now documents of light rather than of the hand. The visual culture of the twentieth century — journalism, cinema, advertising, identity cards, the family album — all passes through the threshold opened in that Paris academy session of 1839.