The American long jumper did sense, though, that his athletics career would be fulfilled, his life changed for ever in that one moment. The photographer Tony Duffy, actually a 'not very good' accountant by trade, was left unaware that a new world was about to open up.
Yesterday the two men were reunited in London at the launch of a book, Visions of Sport (Kensington West Productions, pounds 14.95), a product of one of the world's top sporting photographic agencies which sprang from that moment.
Duffy was an athletics fan who dreamt of being a sports photographer and, with his new lens to make himself look professional, he had made his way to the Olympic Games of 1968. The stadium was still nearly empty for the first event of the day so he gatecrashed the front-row seats.
To his and the world's astonishment, Beamon came and went, Duffy snapping away. He then took his film to a laboratory in Mexico City and left it for a couple of days. One picture leapt out - literally - but only when he got back to England did he do anything with it, sending prints to magazines. Amateur Photographer was the first to use it, paying pounds 25.
Duffy realised the potential as more and more publications requested it and he registered his own photographic agency, Allsport. Now, 50,000 copies of the Beamon picture later, it is one of the world's largest, the new book a celebration of its spectacular work.
It is introduced by the Beamon picture, in which the astonishing height he reached is captured, the concentration in the eyes leaping out. His jockstrap is even visible. 'He hung in the air like Michael Jordan going to the basket. I remember seeing the whites of his eyes,' Duffy said.
Beamon's recollection of one of sport's most memorable moments - a world record that smashed to smithereens the previous 27ft 4 3/4 in of his compatriot Ralph Boston, stood for 23 years until Mike Powell broke it by two inches in Tokyo and remains an Olympic record - is also still vivid.
'Going down the runway I was between time and space,' he said. 'I did not hear anyone talking. It was just me. It was instinct. It was drama. In landing, I landed on my bottom instead of two legs and the impact was so incredible I felt like a piece of rubber. But it was nothing out of the ordinary. Everything was very normal.'
He had to wait 45 minutes while it was measured and re-measured. 'Suddenly it just slapped me in my face. I wasn't here, I wasn't there. I was just flabbergasted.'
Beamon made his own Black Power gesture after receiving his gold medal, wearing black socks with his tracksuit trousers rolled up, but was never, he says, subsequently subjected to the same racial abuse as Tommie Smith and John Carlos.
He has, however, gone largely unremembered in his own country, where track and field was a minority sport that became more minor in the 1970s. He now works for the Recreation Department of Dade County, Miami. The once wayward New York City street kid has also recently set up the Bob Beamon Foundation for Youth to help, morally and financially, deserving sporting cases.
He might also have another plan. The World Championships are in Mexico in 1997, he was told. 'Well, I will probably start training again,' he said.
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