Kodak moments are nowadays the province of pixels

Eighteen advisory editors, under the direction and editorship of the historian Robin Lenman, have taken four years to prepare this staggeringly large lexicon of the 165-year history of photography. It is tempting to suggest that a critic will be hard put to find anything that has been omitted from The Oxford Companion to the Photograph . The scope is breathtaking, with more than 1,600 entries arranged alphabetically and more than 800 biographical entries. There are regional and national entries seldom featured in histories and encyclopaedias of photography: on Africa, Oceania, Scandinavia, Japan, China and South-East Asia, for example.

Furthermore, as the editor notes, "another distinctive feature of the book is its strong emphasis on photography as a social practice". He explains:

"By the year 2000 there were few households in the developed world without at least one camera, and billions of images were created around the world, not only by private individuals, but by scientists, doctors, detectives, immigration officials, and private and official agencies of all kinds."

Picture-making has developed into a huge manufacturing and research industry, nearly every aspect of which is covered in this book.

Its conception began shortly before the millennium. By then, digital photography was firmly established, although purists continued to resist change, claiming correctly that digital images were markedly inferior to silver-based ones. But now, only six years later, digital imaging has conquered nearly every branch of professional and consumer-oriented photography. Of course, photography underwent earlier revolutionary changes - improvements to the daguerrotype and calotype processes in the 1840s, the wet-plate process in the 1850s, dry plates in the 1870s, the half-tone photomechanical reproduction process and the roll-film camera in the 1880s, the 35mm camera in the 1920s and the 35mm colour film in the 1940s and 1950s, followed by waves of automation in the last third of the 20th century. However, Lenman notes: "The speed of the digital revolution took nearly all professional commentators by surprise, and its potential scope and impact on other technologies make prophecies about future trends problematical." He has therefore wisely limited this book to outlining the principles of the early history of digital imaging and printing while including a more general article on the history of photographic innovation.

Numerous other technical entries in the Companion range from "Adobe Photoshop" to "Zone plate", from "Lens development" to "Three-dimensional photography", and from "Fourier optics" to "Optical transfer function".

More obscure technical entries include: "Fresson process", "Gavassian optic", "Photoblogging", "Scheimpflug rule" and "Schlieren photography".

The photographic act has always had strong emotive connotations. None more so than in the distribution on the web of explicit images of child pornography. "This last area particularly, which exercised the British photographic press in the autumn of 2004, is likely to fuel debates about civil rights, privacy and freedom of expression for years to come," Lenman notes. I am sure he is right. Ditto photography's political connotations, with controversy ever since the 19th century about the role of the camera in politics and war. Thus there are entries on "Propaganda", "American civil rights movement", "Struggle photography" in South Africa, and even the horrors of the My Lai massacre in Vietnam and Wehrmacht atrocities in the Second World War.

The biographical entries are deliberately limited to fewer than 1,000 individuals. The giants are there - Henri Cartier-Bresson, Louis Daguerre, Joseph Niepce, Alfred Stieglitz, William Fox Talbot and others - but so are details of living photographers such as Nan Goldin, Andreas Gursky and Cindy Sherman, and also iconic figures such as the Canadian photographer and print-maker Peter Pitseolak and amateur photographers such as the painter Edgar Degas. Other important figures appear in the survey articles and can be located via the index.

Despite its being a reference book, the Companion contains much excellent writing to lure readers into appreciating its literary merit. As for the illustrations, the choosing must have been a near impossible task. Many of the images seem never to have been published before, for instance: Lionel de Rothschild's portrait of his fiancée, Marie-Louise Beer (1912), Pierre Bonnard's Marthe in the Garden (1900-1901), Emma Barton's Old Familiar Flowers (1919), Daido Moriyama's Yokosuka (1970) and Ian Waldie's enigmatic photograph of Diana, Princess of Wales, surrounded by paparazzi on the day that her marriage ended. But there are plenty of classic images too, such as Robert Capa's The Falling Soldier ( Death in Spain ) (1936), Julia Margaret Cameron's albumen print Iago (1867), Cartier-Bresson's sheltered punter in Ascot (1953), Man Ray's Noire et blanche ( Kiki with African mask ) (1926) and Dorothea Lange's Migrant Mother ( Prairie Mother ) (1936).

At the beginning of the 21st century, one realises that photography is on the threshold of a new era. Long-established cameras have been discontinued and the camera-equipped mobile phone, capable of taking pictures and sending them anywhere in the world, has arrived. There is increasing integration of the photographic, electronics, computer and telecommunications industries. It is impossible to predict the future, except to be sure that digital imaging will dominate photography. This invaluable book, coming as it does at a watershed in the medium's history, not only reminds us of the past, it goes some way to prepare us for the years ahead.

Christopher Ondaatje is a trustee of the National Portrait Gallery and the author of many books illustrated with his own photographs, the latest of which is Woolf in Ceylon .