Neal Slavin took this picture of pugs and their owners in 2005 for an advertising campaign. The Brooklyn-born Slavin was by then a master of the group portrait. His book When Two Or More Are Gathered Together first came out in 1974 and featured bodybuilders and gravediggers, hotdog vendors and firefighters, a unique collective portrait of Americans (which he followed up with Britons a few years later).

His vocation came to him, he says, when he was looking through some family portraits and came across a picture of a scout troop. “Some were laughing, some clowning, some gazing solemnly into the camera lens, but all exuded an extraordinary humanness, a unique individuality. I wondered about each boy, about what happened after that photo was taken.”

Slavin is now 83. An updated 50th anniversary edition of When Two Or More… will be published later this year. Speaking to me recently from New York, he described how he liked his groups to find their own composition. “People assume I arrange everyone and get back to my camera,” he said. “It’s actually the opposite. When I photograph a group, I call myself a half sociologist, half photographer. In every group, there’s different hierarchies. There will always be one guy with his chest puffed out, and another who just wants to hide. I try to let them be themselves.”

The pugs picture was a case in point. “We’d built this sort of step affair for the dogs,” he says. “And I decided that they should have a royal treatment with the red velvet curtains. But it wasn’t really working. The owners were hidden behind the curtain with their dogs on leashes. But then it hit me like a ton of bricks. Why are we hiding them? I asked them to push the curtains aside. And there was the picture. After that, it took five minutes.”

• A 50th anniversary expanded edition of When Two Or More Are Gathered Together is published in October by Damiani (£35)