WHAT IS A PORTRAIT? It's a good question, and not necessarily one you'd expect to be raised by an exhibition of photographs from the files of National Geographic, a publication known -- perhaps unfairly -- for its pictures of places rather than people.

All the same, the magazine is the source of a show that asks just that, albeit obliquely. On view at the National Museum of Natural History and accompanied by the publication of a book, "In Focus: National Geographic Greatest Portraits" walks right up to the "edges of what we'd consider a portrait." That's at least according to Leah Bendavid-Val, who edited the lavish, handsome book.

The one common element here is, of course, the pictures' human subjects, although, as Bendavid-Val is quick to acknowledge, even they sometimes function more as props than as focal points. Take, for instance, Willard R. Culver's somewhat campy 1940 photograph of the largest pneumatic tire ever made at the time, a giant doughnut standing more than 10 feet tall and tipping the scales at 700 pounds. Yes, there are several pretty women in the shot as well, but they're about as integral to the scene as the cheesecake hired to stand next to the cars at auto shows. Similarly, the human face -- which we tend to expect front and center when looking at something billed as a portrait -- isn't even essential, as we learn from Thomas J. Abercrombie's 1968 photograph of a red-veiled Afghan woman with a birdcage balanced atop her head.

There is, in short, no final answer to that question.

This being National Geographic and the National Museum of Natural History (as opposed to an art museum), the edges of the definition of portraiture are only nudged, and not knocked over. As much a history of a magazine -- and a conservative one at that -- as it is a history of photography, "In Focus" is organized chronologically by a few broad and, as it turns out, overlapping themes.

Ambiguity, for example, identified in the wall text as a hallmark of the photography of the 1990s and beyond, is actually a quality that has been present since photography's earliest days. It is, however, something that audiences of today are not just more comfortable with than ever before, but something that contemporary photographers, even those who work for National Geographic, increasingly seek.

The return to "realism" so touted in the photographs of the 1970s and 1980s (this after a kind of Pollyanna escapism that characterized much of the 1930s, 1940s, 1950s and 1960s) is nonetheless not the warts-and-all documentation of, say, Nan Goldin's bohemian New York demimonde, even at its grittiest. If anything characterizes a National Geographic photograph, it is its emphasis on the dignity -- one might even say the nobility -- of its human subjects, even when wracked with great emotion or trapped in squalor. "You don't say anything bad about somebody," said longtime editor Gilbert M. Grosvenor about his magazine's mission, "and you don't publish embarrassing photographs and information on people."

There is no better example of the endurance of this philosophy than William Albert Allard's 1982 portrait of a young Peruvian sheep herder mourning the deaths of his charges under the wheels of a hit-and-run taxi. It's moving without being voyeuristic. James L. Stanfield, on the other hand, probably comes closest to satisfying the contemporary art world's appetite for the "abject" with his double portrait of a female coal miner from Virginia and her 15-year-old bride-to-be daughter. The posture of the teenage girl, clad in a wedding gown and holding her head in her hands at the edge of her bed as her hard-hatted mother turns away to light a cigarette, bespeaks some obvious, yet unstated, tension.

Despite the picture's obscured faces, it answers the question posed at the top of this review by being a portrait in the truest sense of the word, not by portraying some far-away locale or exotic, quaintly dressed people, but by documenting the rocky interior landscape of the human heart.

IN FOCUS: NATIONAL GEOGRAPHIC GREATEST PORTRAITS -- Through Jan. 2 at the National Museum of Natural History, 10th Street and Constitution Avenue NW (Metro: Federal Triangle). 202-633-1000. (TDD: 202-357-1729). www.mnh.si.edu. Open daily 10 to 5:30. Free.

William Albert Allard's 1982 portrait captures a Peruvian boy mourning his sheep after they were killed by a hit- and-run taxi driver.