''I don't know what made a good picture," said the great photographer Ed Clark. "I never did know. I made a lot of them. But I never figured that out." In Clark's case, it was pure visual instinct: he looked at what was happening in front of him, recognised its aesthetic qualities and just snapped it. That's what happened in 1945, when he was sent to Warm Springs, Georgia, to cover the funeral of Franklin D Roosevelt.
The place was swarming with lensmen: 135 of them at a conservative count, all penned together in front of a little house. Give or take a wide-angle lens or so, they would all take the same picture, of the ammunition wagon bearing FDR's coffin. Then Clark heard, somewhere behind him, the whining melancholy of an accordion. He turned and saw an amazing sight: a black US Navy officer, in uniform and cap, was playing the instrument, with two lines of tears streaking his cheeks. His eyes were turned heavenwards, and the deep, deep sorrow of his heart was eloquently written in them. Nearby, some white women, two with arms folded, looked at him with disapproval, as if he had no business being there, or behaving like that. But the nobility in Graham Jackson's face put all of them to shame. Ed Clark foregrounded him with his camera, like a hero. "I thought to myself, 'My God, what a picture'," he said later. "I was the only one who saw it."
That sense of having a private secret, denied to the rest of the world, is a constant virtue of Clark's work. He snapped the perfect moment when a tiny Caroline Kennedy, aged a few months, peeped over the crepe lining of her cot at her smiling father, JFK. He snapped a quiet backstage moment during the filming of Gentlemen Prefer Blondes when Marilyn Monroe and Jane Russell, two of the world's most beautiful women, were taking a break in figure-hugging sequinned gowns, behaving like ordinary girls – the former swigging a Coke, the latter adjusting her maquillage – while retaining an unloseable grace.
That's life, Clark might have said, the seemingly unimportant bits captured when the rest of the world wasn't looking. And what distinguishes the great LIFE photographers – celebrated in a spanking new 600-page anthology – is their humanity. Whether photographing war zones, lovers in the park, opera-house crowds or famous faces caught off-duty, they contrived to involve the viewer in a shared revelation of human character. Is there a clearer embodiment of the nation's shock at the assassination of JFK than the face of the woman – serious, well-dressed, expensively coiffed, utterly aghast – in New York City in 1963? Is anything more certain than that the sweet boy in the Dodgers T-shirt, snapped by George Silk, will grow up to be the pot-bellied, bow-tied, cigar-smoking image of his grandfather?
Sometimes the photographers employed secret devices to catch humanity unawares – like Yale Joel, who rigged up a two-way mirror in the doorway of a Broadway cinema in 1946, and caught unsuspecting passers-by responding to their own reflections. Some seem puzzled by their mirrored selves, some inspect their stained teeth or ill-fitting waistband – but all of them look like people putting on a little show for their private selves.
LIFE had three distinct incarnations in its history. It began in 1883 as a general-interest magazine, full of social comment, jokes and Norman Rockwell paintings. In 1936, it was bought by Henry Luce, publisher of Time, for $92,000, and re-launched later that year as the first photo-journal. Its devotion to the image was total. Beneath the photographs, lovingly reproduced on heavily coated paper over 50 pages, the text was reduced to captions. The cover price was a mere 10c, and its circulation hit a million inside four months.
The new anthology chooses images from the magazine's heyday, from 1936 to 1972, and many of the images that achieved classic status are from the Second World War. Time and Life sent 40 war correspondents, and every week featured photographs from the conflict, educating their readers in a new aesthetic of all-too-real-ism. Photographers learnt the possibilities of their craft under the most extreme conditions. Robert Capa, who was among the first wave of troops on Omaha beach in June 1944, famously said, "If your pictures aren't good enough, you're not close enough," a dictum that sent many of his co-professionals ever nearer to the action.
W Eugene Smith, who covered the war in the Pacific, was known as "Wonderful" Smith by his soldier colleagues for his cavalier disregard for his own safety. "He always wanted to be in front of the first soldier in combat," said one. His shot of marines blowing up an enemy blockhouse at Iwo Jima makes us feel the blast practically rasping our cheeks. Another familiar Life image was the picture of a procession of GIs advancing past the recently dead body of a colleague. George Silk took the classic shot – but he was also responsible for the beautifully composed portrait of a dead German soldier in Holland in 1945, his hand eloquently resting on his hip, as if he were a shepherd boy asleep in a cornfield.
In a perfect photograph, form and content seamlessly combine. Life photographers often came up with unusual or dramatic compositions that made sense of chaotic reality. In 1952, the great Margaret Bourke-White took an aerial shot of a dramatic event on the beach at Coney Island, when a woman nearly drowned – and showed humanity forming into a whirlpool, a storm of sympathetic interest, in which the stricken swimmer was the eye. Herbert Gehr's camera seems to hover over Times Square to capture – in perfect film-noir shadows – the three sailors on a drunken spree in a deserted New York. You can practically hear the soundtrack of On the Town or Guys and Dolls. How did he do that? And the pioneering Gjon Mili, with his stroboscopic camera that could take pictures in 1/100,000th of a second, froze gymnasts, tennis players and lindy-hop dancers in mid-air, to make time stand still.
To freeze time, to capture the moment, to put the viewer slap-bang into the action, to make the everyday astounding, to celebrate the beautiful and record moments of giveaway tenderness in human lives – that's what Life magazine did in its 36-year reign as the crucible of photojournalism. Its practitioners were masters of self-effacing discretion. "Much of a Life photographer's energy was spent trying to remove from the scene any trace of his or her presence," writes John Loengard in the book's preface. "[They] often focused on human expression and gesture. These might be coaxed from a subject, but never demanded. Most often it was simply a matter of waiting. Subjects became bored. Their minds turned to things they found more intriguing than the photographer sitting in the corner. Snap!"
'The Great LIFE Photographers' is published by Thames & Hudson, £16.95, out nowReuse content