Family photos in Japan, especially ones taken for formal occasions such as shichi-go-san (seven-five-three) ceremonies, are often as stiffly posed as 19th-century tintypes, with Mom, Dad and Junior never cracking a smile.

When American photographer Bruce Osborn began taking photos of parents and children in 1982 — his first was of an angelically grinning mom and her casually affectionate punk-rocker son — he started a small revolution that continues today, more than three decades later.

The first thing to go in Osborn’s “Oyako” (“Parent and Child”) series — more than 4,500 entries to date — was the stiffness. Instead, Osborn captured a natural affection and uninhibited playfulness that may look strange to those who think of Japan as the land of the samurai and their stone-faced salaryman descendants, but nonetheless represents something real, as well as universally human.

“Back when I started there was a lot of talk about the generation gap between parents and children,” says Osborn, now an ebullient 63. “So when I found out how much the generations really had in common, it was kind of a surprise for me.”

Born and raised in Southern California, Osborn began his career shooting the Los Angeles music scene, and his early oyako photos were largely of people in the arts and entertainment world and their parents (or vice versa). But he has long since branched out, photographing everyone from geisha, sumo wrestlers and firemen to Tohoku residents shortly after the March 11, 2011 triple disaster.

The series, which started life in a punk-music magazine, has also appeared in a wide range of media, both print and online, as well as being presented in gallery exhibitions and photo books. Now there is a documentary, “Oyako — Present to the Future,” by veteran TV-commercial director Toshiro Inomata that is screening at the Tokyo Metropolitan Museum of Photography in Ebisu through June 4.

What the film shows, in a variety of situations, is Osborn’s talent for drawing people out, including those who may not have a lot to smile about, such as Tohoku disaster victims.

“Taking photos is like a jam session,” he says. “It’s about being in the situation, in the moment. Most people come into the studio a little nervous, so the first thing I have to do is get them relaxed and moving. I start a little back-and-forth to get them out of their shell and find a commonality of some kind.”

Once established, that connection can last for decades: Osborn recently photographed his original oyako after a gap of nearly 30 years, with the now middle-aged punk rocker still sporting a mohawk. “I’ve taken a number of people over a period time, say every 10 years or so,” he comments. “By now it’s a whole series.”

Osborn’s photos can also change lives. In the film, the son of a former sumo wrestler tells how an oyako photo taken when he was a boy led him to work with his father at his chanko-nabe (sumo-wrestler-style stew) restaurant. “It can become very personal for people,” Osborn says. “The photos show what you are. They present your relationship to your family.”

The oyako series, which Osborn describes as “my life’s work,” has now taken on a life of its own with what he dubs Oyako no Hi (Parent and Child Day). Since its start 12 years ago, Osborn and his supporters have been making the fourth Sunday in July a day for oyako-related events, including a massive one-day photo session with 100 oyako conducted by Osborn and his team. Families interested in having their photo taken can apply at www.oyako.org.

“I’m surrounded by so many people when I shoot that I feel like E.T. when he’s in the closet with all those dolls,” he says with a laugh. “I’m just kind of peeking out.”

The influence of Osborn’s project, however, now extends beyond Oyako no Hi. He has been invited to speak to members of a national portrait photographers association and has even inspired some of them to adopt his methods. “Very often Japanese (photography) studios have been in the same family for three or four or five generations,” he says. “It’s a very conservative business, but some of the younger photographers are approaching family portraits in a new way.”

Over the years, Osborn has also seen changes in Japanese families themselves. “The biggest is that families are becoming smaller units,” he says. “Also, people are very busy now. Most mothers have part-time jobs and most kids have after-school activities. On the other hand, fathers are much more involved in their families than in the past. They’re taking a more active role in their children’s lives.

“It’s hard to talk in generalities about families,” he concludes. “That’s what keeps me interested in doing this. Each family is unique.”

“Oyako — Present to the Future” is showing through June 4 at the Tokyo Metropolitan Museum of Photography (03-5420-3080; www.syabi.com).