THE QUIET, understated photographs that artist Nicholas Nixon has been taking of his wife, Bebe, and her three sisters annually since 1975 -- on view at the National Gallery of Art -- may have their roots in family snapshots, as curator Sarah Greenough says, but they're not like the pictures in your photo album.

For one thing, Bebe (she's the one second from the right in each of the 31 black-and-white prints) and her sisters, Heather, Mimi and Laurie, rarely smile. For another, you don't and can't possibly know these people. As inscrutable as they are, they might as well be buildings, like the ones in Alabama that artist William Christenberry is known for returning to year after year and photographing in states of ever-increasing decrepitude. The sisters have never spoken publicly about the series. "In a way," Greenough says, "it draws us in more."

But what do we find, the deeper we go?

To be sure, the creases and crow's-feet become more strongly etched on the face. Fashion don'ts become less embarrassing over time. But what can we learn, or guess at, about what goes on inside their heads from the subtly shifting external clues? Here Mimi is pregnant, as Heather tenderly touches her belly; there Laurie crosses her arms, in a bit of cryptic body language, as she does in three of the earliest shots. In this one, the sisters embrace in an evocation of single body with four heads; in that one, they barely touch.

The temperature of Nixon's photos of the Brown sisters straddles the documentarian's detachment and the emotional intimacy of a family member. They are neither entirely cool nor entirely hot, but somewhere in between. In a way, the experience of looking at them is not that unlike looking at our own photo albums.

That's because, in the end, it is not the Brown sisters we end up seeing on the wall, but ourselves, reflected in their strangely sad and slowly old-growing eyes.

NICHOLAS NIXON: THE BROWN SISTERS -- Through Feb. 20 at the National Gallery of Art, West Building, Constitution Avenue at Sixth Street NW (Metro: Archives-Navy Memorial). 202-737-4215 (TDD: 202-842-6176). Open Monday-Saturday 10 to 5; Sundays from 11 to 6. Free.

National Gallery of Art staff lecturer Sally Shelburne will present a gallery talk about the exhibition on Nov. 29 and Dec. 4 at 1 and on Dec. 3 and 13 at noon.

"Nicholas Nixon: The Brown Sisters," featuring portraits of the artist's wife and her sisters taken over the years, goes beyond face value to capture the intimacy of family and more. The exhibit is at the National Gallery of Art.