With “National Gallery,” documentary helmer Frederick Wiseman expands his focus from chronicling the day-to-day of U.S. institutions to include the famous London art museum. It is his third film to play in Cannes, after “The Last Letter” and “Boxing Gym.”

Variety: Were you aware that you happened to be approaching the National Gallery at a moment when they were reevaluating their own identity?
Wiseman: I was just lucky, because I didn’t know any of that. So much of what you come across in all these movies is chance. I’ve wanted to do a museum for a long time, because I thought a museum would fit very well with this institutional series I’m doing, I just hadn’t gotten around to it. I tried informally at the Museum of Modern Art in New York, but I was advised informally that I would not be able to shoot it there. And I tried maybe 25 or 30 years ago at the Met, but they wanted to get paid, and I didn’t have the money to pay them. I’ve never paid anybody.

Variety: You’ve been free to make the films you want without interference for the past 50 years. How does the business side of your operation work?
Wiseman: Except for one glorious 10-year period between ’71 and ’81, where I had two five-year grants from the Ford Foundation earmarked for me to make one film a year, I’ve had to sing for my supper. I get the money for each film from some combination of the following sources: PBS, ITVS, sometimes National Endowment of the Arts. I’ve gotten money from PBS for I think every film but “Crazy Horse” — and that was a film where I think PBS would collapse if “Crazy Horse” were on it. I try to get enough money to pay for the production so if it’s sold afterward, that gives me something, but it’s rare that I get a full budget that includes a salary for me. In America, you can make more money talking about movies than making them, so I do the lecture circuit.

Variety: The film runs three hours, but doesn’t feel obliged to reflect the National Gallery’s collection, the way other museum docs often do. How would you describe your focus?
Wiseman: I have 170 hours of rushes, and the film is three hours, so that’s a ratio of 55:1. I always have to shoot a lot. You never know what you’re going to get. The real metaphor for the whole thing is Las Vegas. You have to take chances all the way through. It varies from 70 hours to 250 hours. (My previous film) “At Berkeley” was 250, apparently because academics do like to talk, and once you go into a meeting or class, you have to shoot the whole thing. The worst thing you can do is anticipate when the good moments are going to arrive, because there’s only one rule in this kind of filmmaking and that is if you try to anticipate and turn the camera off, that’s almost inevitably the magic moment.

Variety: What kind of research or preparation do you do before filming, or does it help to go in somewhat naive?
Wiseman: I never know what to do to prepare other than to get a sense of geography of the place, and to get some sense of the daily, weekly or monthly routine. So that’s all I do in advance, I walk the boundaries of the place. It’s often never more than a couple buildings, and then I talk to people about the routine: Is there a weekly staff meeting? Is there a meeting between the director and the various heads of departments?

Variety: Was “National Gallery” easier to make than something set in a more volatile environment, such as “High School” or “Near Death”?
Wiseman: Each film presents its own political problems. In my experience, it has been relatively easy to get permission. It’s rare that I’ve been turned down, and only once have I actually started to shoot and then had to stop. I started to do a film on the police, a film that became “Law and Order,” and after a week shooting in Los Angeles, I was told I could do anything I wanted except ride around in the police cars, and at that point in Los Angeles, there were no foot patrols, so it severely limited the story. I went to Kansas City and shot the film there.

Variety: Have you ever considered revisiting one of the institutions you documented in the past to see how it has changed?
Wiseman: No. Each movie is a new adventure because I’m going to a place where I haven’t spent time before, the intellectual issues are different, you meet people from all walks of life. Even though I’ve made a few movies outside of America, one of the things I think I’m doing is having a look at contemporary American life. It’s basically a look at the way we live through the framework of institutions that are important to us: education, hospital, the Army. You can never cover it all.

Variety: What institution haven’t you done that you’d still love to document?
Wiseman: I’d love to do the White House. If anybody gets me permission, I’ll start tonight.