Photography as a popular activity is usually dated from the appearance of the first cheap cameras in the late nineteenth century. Its intellectual origins, however, are considerably older. For at least six hundred years, painters, astronomers and mathematicians had been experimenting with devices that produced faithful images of the outside world on internal surfaces. The history of these experiments offers a useful reminder that photography did not appear suddenly but was the end point of a long accumulation of optical and chemical understanding.

The camera obscura - from the Latin for 'dark room' - was the earliest such device. Its principle is simple: light entering a small hole in the wall of a darkened room produces, on the opposite wall, an inverted image of whatever lies outside. This phenomenon was noted in ancient China and in Arabic optical treatises, and the eleventh-century scholar Ibn al-Haytham gave it a systematic description in his Book of Optics. From the fourteenth century onwards, European astronomers used the camera obscura to observe solar eclipses, because looking directly at the sun was known to damage the eye.

In the sixteenth century, Italian painters adapted the camera obscura for their own use. A lens placed in the aperture sharpened the image considerably, and practical devices shrank from room-sized to table-sized. Painters could then trace the projected image on paper and use it to establish the proportions of a scene. Art historian Dr. Margareta Ilves has documented the likely use of a camera obscura by the Dutch painter Vermeer, based on distinctive optical features in his work: the slightly soft focus of distant areas, the characteristic highlights on surfaces, and the accurate but unusual perspectives in certain interiors. Her interpretation is contested by some specialists, who argue that Vermeer's accuracy could have been achieved by careful eye alone; but the case for mechanical aid, though indirect, is strong.

Throughout the seventeenth and eighteenth centuries, the camera obscura became a widely available tool. Portable versions with ground-glass screens allowed sketches to be made from it in the field. It was marketed as an aid for drawing, for teaching perspective, and for entertainment; in some London coffee houses, visitors paid a small fee to look into a camera obscura projection of the street outside. What it could not do was produce an image that persisted after the light was removed.

The step from temporary image to fixed photograph required chemistry. Several substances had been known since at least the seventeenth century to darken on exposure to light; the physician Johann Heinrich Schulze, in 1724, showed that silver salts were particularly sensitive. Nearly a century later, in 1822, the French inventor Nicephore Niepce produced the first photograph using a pewter plate coated with bitumen of Judea, exposed for several hours in a camera obscura. The image was barely discernible and required enormous exposure times, but it persisted.

Niepce's collaborator, Louis Daguerre, improved the process over the following fifteen years, producing by 1839 the daguerreotype, a much sharper image on silver-coated copper plates. An English rival, William Henry Fox Talbot, developed at around the same time a paper-based process that could produce multiple prints from a single negative. This second approach, the calotype, proved more influential in the long run, because it underpinned the commercial photography of the later nineteenth century.

By the 1890s, photography had become accessible to ordinary consumers through machines such as the Kodak roll-film camera, which required no technical skill and sent exposed film back to the factory for processing. The slogan 'You press the button, we do the rest' summed up the transformation. Meanwhile the older camera obscura, now a curiosity, continued to be built in a few locations as a tourist attraction. A handful still operate today, including the Camera Obscura in Edinburgh and the Great Camera Obscura on San Francisco's Ocean Beach.

Photography, it is sometimes said, changed the way people saw the world. Dr. Ilves has argued that part of the change was simply the extension to the masses of a way of seeing that had long been available to painters and scholars. For centuries, the camera obscura had shown that a flat image could capture a scene with startling accuracy; photography merely provided the chemistry to keep the image after the light had gone.