Magnum photographic agency began life 60 years ago. A few profoundly gifted individuals possessed of lofty ideals founded an agency that was to be politically engaged, liberal, humanistic and serious. When Henri Cartier-Bresson, George Rodgers, Robert Capa, David “Chim” Seymour and Bill Vandivert cemented the idea over lunch in the penthouse restaurant of the Museum of Modern Art in New York in April 1947, Magnum was to be a bastion of 20th-century photojournalistic values.

You can see how magnificently it succeeded in a new book published by Thames & Hudson, marking the 60th anniversary of the agency. Magnum Magnumis a great slab of a book – 40cm x 33cm (16in x 13in) and weighing almost 7kg (15lb) – containing more than 400 photographs reproduced on a lavish scale and with equal quality, from Josef Koudelka’s photograph of a prowling hound taken in Sceaux Park in France in 1987 to Cornell Capa’s intimate close-up of Marilyn Monroe and Clark Gable on the set of The Misfits in 1960. Each of Magnum’s 69 photographers is represented by six works, chosen by another member, with a brief text.

The selections are fascinating and often eccentric, and this provides part of the joy of the book; but the cooperative nature of the project is also symbolic of the original aims of Magnum’s founders, which was to have an agency owned and run by the photographers themselves, who would retain the copyright in their images. To this day, it is fully owned by its members.

Eve Arnold, one of the earliest members, has selected six works by Cartier-Bresson, explaining in her text how he taught her of the need to try to tell an entire story in a single definitive image. His picture of refugees exercising at the Kurukshetra camp in Punjab 1947 is typical, managing to pack in large gestures, precious camaraderie and bitter loneliness. It is like the silent equivalent of an epic drama.

Magnum’s early members all had outstanding eyes, brains and charisma. Their principles were admirable. They believed in human dignity and the capacity for improvement. They were almost like a family with very high moral and ethical standards. And they adhered to certain rules: they allowed no cropping; they wrote their own captions; and they were self-starting, devising their own projects independently from the commissioning processes of magazines.

Philip Jones Griffiths has been with Magnum since 1966, when he began a five-year period photographing in Vietnam. He covered the Yom Kippur War and then worked in Cambodia between 1973 and 1975, and since then he has taken himself on assignment to more than 100 countries. Ian Berry, who selected from Griffiths’s work, has noticed that human foolishness attracts his eye. Among his choices is a picture taken in Belfast in 1973 showing a housewife mowing her lawn while casting a wry glance at a British soldier crouching beside one of her flowering shrubs.

Griffiths has selected pictures by Ian Berry, including a lovely portrait of some men in an open boat in monsoon rain in Bangladesh, taken in 2000. Griffiths writes that Berry was the first Magnum photographer he met. “He epitomised everything I thought a great photographer should be – egoless, self-effacing, always present but never seen.”

Magnum started out so well, and its standards and reputation have always been exceptionally high. The best photographers have generally become Magnum photographers, and this is how the agency has stood out for so long in a world increasingly crowded with photographic agencies. But over time Magnum has struggled to maintain the original vision of its founders. Three of the first seven members were dead within ten years of its beginning. New members came in who did not see eye to eye with the founders. Membership has grown and become much more diverse. Art photography now sits alongside photo-journalism.

No longer do Magnum members devise their own projects; they tend to work like any other agency photographers, accepting commissions and carrying them out. They crop their work. Some make alterations using computer technology. They don’t necessarily write their own captions. But the biggest change is that the need for a photo agency has altered dramatically in this digital age. Millions of photographs can now be retrieved by computer. Magnum is having to redefine the way it functions.

All of which perhaps explains why Magnum has chosen to mark its anniversary not with an exhibition or a film but with a book, a traditional, tangible object that can be handled and accessed in the same way that photography books were handled 60 years ago. Magnum Magnum is beautiful and pleasurable to leaf through but in the end, the book, and the traditions it entails, look backwards not forwards.

In lauding the agency’s achievements based on its original principles, Magnum has unwittingly provided a tangible full stop to its golden era. Which raises the question: what will become of Magnum in this new century?

Joanna Pitman picks Magnum's all-time best images

Stuart Franklin – Tiananmen Sq, June 1989 published in Time. This picture shows one defiant Chinese student blocking a line of tanks in Tiananmen Square.

Marc Riboud – A girl with a flower confronts the US National Guard during an anti-Vietnam war march, Washington DC, 1967.

Rene Burri – The iconic image of Che Guevara, when he was Economics Minister, taken in Havana in 1963 and first published in Chicago’s Look Magazine.

Werner Bischof – On the way to Cuzco, Peru, 1954. A young Peruvian boy plays a pipe as he walks beside a valley. This is one of the most famous revelations of grace and beauty in the face of the world’s downtrodden poor.

George Rodger – The photographer succeeded in reaching the isolated Nuba territory in Sudan in 1949. His famous picture of a Nuba wrestling champion carried on the shoulders of another wrestler was first published by Weekly Illustrated in October 1949.

— Magnum Magnum is published by Thames & Hudson, £95

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