It's like when you rent an old family summer house and one rainy day you find their photograph albums in a sideboard.

Children in skewed bathing suits play croquet, parents pose by a Buick convertible (whose? why?) on a blazing lawn, half-smiling men hold up fish and wait for the shutter to click. Is that the troubled son, the one you heard about at the grocery store, tilting his resentful head away from everyone else in the picture? On the porch, a matriarch cradles her latest grandchild with a measured satisfaction while her daughter-in-law stares at her.

You feel a vicarious nostalgia, and a sense of either tragedy or simply the poignancy of time, you're not sure which. These people have the awkward privacy to be found in snapshots, the sort of privacy that confers entitlement -- you feel uninvited, even intimidated. You keep looking. Then you put the album back where you found it.

So too with the "The Brown Sisters," as photographed by Nicholas Nixon. They are on view through Feb. 20 at the National Gallery of Art.

Nixon, a polished art photographer, has borrowed the old summer-house snapshot style to make black-and-white pictures of the four sisters with an 8-by-10 view camera, on sunny days or cloudy, cold weather or warm, front- or backlit as if the pictures were governed by no more rules of composition than snapshot happenstance. Sometimes, in coy clumsiness, the shadow of Nixon and his camera appears on his subjects.

Always in the same order, they stand shoulder to shoulder on lawns or beaches, as if they'd just been rounded up after lunch and told that it was time for the yearly picture. (Much groaning as they push back their chairs. Requests to wear makeup, fix their hair, all denied.) Nixon is said to take many pictures each year, but, snapshot-style, he has chosen only one a year for 31 years. The series began in 1975 with the youngest at age 20, plus or minus a few years. It has taken them now into middle age.

Four sisters: sweet-faced, wide-smiling Heather; confident, tennisy Mimi, who might be beautiful if she weren't lacking mystery; wistful, wary Bebe (Beverly, who is married to Nixon); and the unsettling Laurie, her poses oblique and theatrically haughty. You feel as though you're looking at them through a hedge.

Why does tanned Mimi, with her bold, clear, can-do face, cut her hair off every 10 years or so? And then, her look of purpose suddenly gone, why does she seem to retreat behind the others in the last pictures? Why does Bebe go from the radiance of her twenties to the embarrassed wariness, even fear, of her middle age? And Heather -- how does she come to assume the matronly authority of a dean hired by some school to deal with peckish alumni? Why does Laurie cultivate the desperate severity of a Johnny Cash album cover?

In one picture or another, they all wear wedding rings, I believe. None wears much else in the way of jewelry. Weight is gained and lost, especially by Laurie. There's at least one pregnancy. They dress as if they were about to walk the dog, except for one early picture when they all wear summer dresses, as if attending a boarding school reunion. All come equipped with what might well be admired as the Brown chin, and all but Bebe have an androgyny of the sort that signals respectability among the upperish classes of New England.

Beyond the pictures, we're told nothing about these women. How minimalist, how abstract, how conceptual or postmodern, as if the exhibit isn't about the sisters but about some aesthetic principle or about photography itself. But now that we've paid our homage to critical theory, let's get back to the sisters. They're what's interesting, both despite and because of their opacity, as they change and age, fade and glow, with beauty settling on one face or another depending on the light, the year, a death or a triumph, or just the caprice of the shutter.

In natural light, with little or no makeup, four women are not apt to look beautiful at the same time. And a photographer can make anyone look alienated and bewildered simply by waiting long enough to take the picture -- I suspect this is the shopworn trick that made Richard Avedon's pictures of the powerful and famous so celebrated when they appeared in Rolling Stone in the 1970s. Also, the photographer gets to pick the prints, but if Nixon -- unlike Avedon -- has an agenda, it's hard to tell what it is.

The sisters may well have been brought up to feel embarrassed, to advertise their modesty with a puzzled discomfort when pictures were taken (except for a formal wedding shot). If so, they were also brought up to be polite at such moments, to "put up with it," to "make do." Politeness creates a cruel distance, as every well-mannered person knows so well. When directed at you, as it is here, it makes you feel as if you're invading, breaking some unwritten rule, and therefore you're intimidated.

You have to give them credit -- year after year they put themselves on public show knowing that these are the sort of pictures that make women say: "I always look so terrible in photographs." Or do they understand that they are performers in a work of art, cast in the roles of themselves? Or do they hate Nixon for using their sister Bebe to make them go along? But isn't Bebe a bit of an outsider, too, herself with the blond hair, the bohemian disarray?

Whatever has gone wrong in their lives they seem to redeem in early middle age, only to discover that they have graduated into a world where every day means managing a collection of compromises rather than possibilities. Something bad seems to have happened in 1998, but we don't know what -- some discovery that led them into resignation, as if they'd all gone at once from managing their youth to managing their age.

Again, this could be the whim of the shutter, the Scrabble game they just argued over, who knows? Still, they seem to huddle together more in the later pictures and you sense a tragedy happening here, the universal and animal tragedy of time, no matter what heaven or oblivion you think awaits you.

Nixon has made a name in art photography with his photographs of AIDS patients, sex (balletic and chaste) and cities, powerful and immediate pictures of things distant from us. He chronicles creation and destruction, and he plays with the line between public and private, a line that has grown so squalidly vague in America.

With the Browns he has brought us unanswerable questions that we have to keep asking, stories we keep telling ourselves -- about which sister was jealous of whom and why, which one might have sailed a 40-foot sloop across the Atlantic, which one reached a state of such acute but limited excellence in college that everything afterward would savor of anticlimax, to paraphrase F. Scott Fitzgerald.

You end up not sure if you'd particularly want to know them (except to satisfy your curiosity), and you may be even less sure that they'd want to know you, just another renter in their family place, leafing through their photograph album.

Every year since 1975, Nicholas Nixon took a snapshot of the Brown sisters, Heather, Mimi, Bebe (the photographer's wife) and Laurie, lined up in the same order. Among the photographs on display at the National Gallery are the sisters in 2004, top, in 1984, center, and in 1978.Always face on, shoulder to shoulder, in the same order, for 31 years: At the National Gallery, Nicholas Nixon's portraits of Heather, Mimi, Bebe (the photographer's wife) and Laurie, 1986, left; in 1993, above; and in 2000.