What makes a portrait photographer is what comes through on the faces.

Prentice Hall Polk has spent most of his 82 years as the official photographer of Tuskegge Institute. In 1938 he had a studio in Atlanta, but only one of the pictures in his Corcoran Gallery of Art exhibit -- through May 17 -- was taken there. The rest he found on the Tuskegge campus or in his studio three blocks away.

So often, people having their pictures taken do not relate to the photographer at all. They are part of a social ritual -- the First Communion photo, the wedding photo, the first promotion -- and the subjects seem to feel themselves to be actors in some pageant rather than real people looking at the camera.

Somehow Polk gets them to really look. "They used to call me the crazy photographer," he said in a talk last week. "I would try to relax them when they came into the studio. I'd hum and sing a little. Some photographers you'll find are running their mouths all the time, but that's not the way. You want to make people feel easy."

A father and his two mirror-image sons stare out defensively at the world. They're not happy with the situation, but at least they are reacting.

Everyone is dressed up to beat the band, usually in white. A young married couple gazes remotely, absorbed in themselves. A girl looks down demurely; an older woman faces the camera squarely, solidly. Some are coquettish, some pensive, a few hopelessly opaque.

"That woman with her hands on her hips, I call her The Boss," Polk said. "I found her on the campus and got her to come in to the studio, and I just thought she looked like she could boss anybody, so I had her put her hands on her hips." It became his most famous picture.

It was intelligent of the Corcoran to place Polk's studio work at the show's beginning, because one likes to see what an artist can do in a rigid and restrictive form before going on to the more freely expressive work. It's like seeing a sonnet by a free-verse poet, an academic study by Matisse.

Polk made a whole book's worth of portraits of George Washington Carver, and pictures here show the great scientist in his lab and at his easel. There are portraits of the Tuskegge string quartet, and Tuskegge's Bill Pipes and His Melody Barons, and various groups. And suddenly: Mrs. Roosevelt! She used to turn up in the darnedest places. Here, she's in a small plane, smiling her famous smile. And Will Rogers! Polk's celebrities manage to look as everyday as his other subjects. There is a Walker Evans quality about his work.

Probably the best-known Polk photos are the ones of old working people, some stolidly confronting -- not merely the photographer, but the world itself. It is useful to study the difference between these and the studio portraits.

"When I asked them for a picture, they'd say they didn't have the money. But then I told 'em, 'No, I'll pay you.' I paid a dollar, which went a lot further in those days."

The son of an Alabama field laborer who taught herself to be a seamstress, Polk knows all there is to know about being black and poor in America. Of his early days in Chicago as apprentice to a commercial photographer, he used to say, "I kept my peace with myself by remembering I wasn't just being used. I could learn what I needed from white people."

Once, in Atlanta, a white photographer encouraged him to enter a photography show. He entered five pictures, of which three were hung -- more than the white man's. But when he came to see the show, he was told to take the freight elevator.

"So I took it," he said. "I didn't come there to fraternize with white people. That wasn't my goal. You can defeat yourself with pride."

If anything, he was too modest about his work. He didn't begin charging money for his prints until five years ago. "A woman would come in and say she liked this on and that one, and I'd say, 'Pick out seven or eight and come back in a few days.' And she would, and I'd have new prints and would give them to her."

Now, even his subjects are beginning to realize that Polk is an artist to be reckoned with. He still sees Mildred Hanson Baker, whose 1937 photo in her senior prom gown, designed and made by her mother, has become famous. When they meet in Tuskegge -- where Polk as been living for 52 years -- she always says she can't believe the picture has taken on a life of its own.

Cataract operations last June slowed Polk down somewhat. He has taken only two pictures in the past year, plus a little printing. But he has discovered the lecture circuit, and recently he addressed the International Center of Photography in New York.

"I have front vision," he says, "and I'm going to have another operation to fix up the side vision. Then I can get back to work."