Coney island kid: Harold Feinstein took some of the great photographs of New York at play

The child of Jewish immigrants, his father from Russia, his mother from Austria, Harold Feinstein grew up during the Second World War, yet in the photographs he began taking as a very young man around his home in Coney Island, Brooklyn, there is none of the shadow of war – only the exuberance, sensuality, sunshine and optimism of peace.

"Coney Island was the centre of the world for me," reflects the 81-year-old on his youth. "I loved the rides, the hot dogs – I've never gotten over it." In his early work, the boardwalk is a place of warm, naked skin, handsome, confident young faces, the fantasy world of an adolescent that, very wonderfully, is indistinguishable from reality.

"Coney Island Teenagers" (previous page), shot when Feinstein was himself only 18, is a wet dream of a picture: the nicely made-up honeypot at the centre with her full lips, strong teeth and plucked eyebrows lying back, head reclining on the naked back of one young man, arm around another, while a third, on her left, is either asleep or on the point of ejaculation – or perhaps merely enjoying the music, Rudy Vallée or Perry Como, pumping out of the portable radio cradled like a baby on her breast.

The following year, in "Beach Concert" (opposite, top), the music is live, coming out of the guitar of the bare-chested hunk centre-stage, but his musical gift does not set him apart from the crowd pressing around his back. His chum, out of the picture, is giving him a drag on his cigarette. A curly-haired little girl with her sunglasses askew is getting her head patted. A man at the back is picking his nose. Here is the raucous, easy-going, plebeian hedonism of a country that has got its life back. In another 1950 shot, a fleshy, grey-haired beauty with a ribbon in her hair grins at the camera as her fat-gutted guy, holding her neck in a clinch, downs a bottle of beer. "If This Isn't Love," it's titled, "Then Maybe I'm Crazy". "When your mouth drops open, click the shutter," Feinstein likes to say. These pictures are as fresh as today's milk.

Then he got drafted into the army for the Korean War – and suddenly the shutters come down. The sun, the smiles and the sex drain away. Even the faces disappear – in shot after wartime shot, we are looking at men in a faceless mass, men high above us lugging kitbags through the rainy twilight, a soldier on a wet, wintry road with his back turned, gazing into the mist, rifle at the ready.

Yet even the war didn't turn out so bad for this sunny-tempered snapper. "I always feel I had a very lucky life," he tells me. "For example, I sure didn't want to go in the army: when I was drafted in the Korean War, I wanted to go as a photographer. But luckily they put me in the infantry – luckily because the official photographer was photographing the medal awarding and all the official situations. Whereas, because I was in the infantry, I had a camera around my neck wherever I went – in training, on the troop ship, in Korea."

And the exuberance of his early work was merely hibernating. Demobbed and back in the US, he was welcomed into the New York School which included figures such as Richard Avedon, Diane Arbus and William Klein, and his work was soon joining the permanent collections of the Whitney Museum and the Museum of Modern Art. He got an introduction to the great W Eugene Smith, 12 years his senior. The relationship had its ups and downs but the older man's influence is palpable in the richer complexity that his later work attained. Smith said of Feinstein, "He is one of the very few photographers I have known, or have been influenced by, with the ability to reveal the familiar to me in a beautifully new, in a strong and honest way."

Then, in 1957, at the height of his precocious fame, Feinstein left New York with his pregnant fiancée "because it was too expensive", and went to live in New Jersey. He was a city boy to his fingertips: "For me, being in the country was when I would see the first cow," he says. He k symbolically married the two worlds in a photograph entitled "Sheep Under Clouds" (above): Coney Island clouds and New Jersey sheep, conjoined in the dark room.

During his decades away from the big city, Feinstein became a beloved and highly regarded teacher. "He has a reputation as an extraordinary teacher," says his present wife Judith Thompson, who has been with him for 25 years, "working with young people coming up into photography. People come out of the woodwork all the time and say, 'You taught me 40 years ago and changed my life.'"

The price he paid was that in New York he dropped out of sight. Decades later a collector of 40 years' standing called Jim Fitts visited him at his home in Merrimac, north of Boston, and was amazed by what he found. "I'm driving back from Merrimac," he writes in a foreword to a new retrospective of Feinstein's work, to be published in September, "and my head is swimming." And the hundreds, perhaps thousands of "brilliant" images were "taken by a photographer who, until a few months ago, I knew nothing about".

But for Feinstein the present moment matters more than his lost celebrity. "I love this life," he writes in the new book. "I feel like I am always catching my breath and saying, 'Oh! Will you look at that?' Photography has been my way of bearing witness to the joy I find in seeing the extraordinary in ordinary life. You don't look for pictures. Your pictures are looking for you. Your job is to see… There's an endless and extraordinary reservoir of energy that comes from saying yes. There's infinite nutrition. You are beckoned, and the more you surrender, the more you see."

'Harold Feinstein: A Retrospective' can be pre-ordered through Nazraeli Press (nazraeli.com), priced £40. An exhibition of his work is at Panopticon Gallery, Boston, from 14 September to 30 October. Prints can be ordered through the gallery's website, panopticongallery.com