It was time to take Alfred Eisenstaedt's picture. The famous pioneer of photojournalism had been roaming the darkened chambers of a daguerreotype show at the National Portrait Gallery, but the minute he saw a working photographer he drifted toward the hallway, heading for the light as surely as a plant seeks the sun.

Then he began to set the picture up. He couldn't help it. Glancing automatically at the hard light bouncing off white walls, he settled himself behind an antique rosewood camera that stood in the center on a pedestal.

"You tell me what you want, now. How do you want me?" He spoke quietly, with a trace of his native West Prussia. Dapper and slight, hardly 5 feet 5, he hovered around the glassed-in box camera as though he needed to get his hands on it.

"I think better when I have my camera," he muttered. "I don't get nervous then."

What you noticed were his eyes, intelligent and quick, and so bright you couldn't believe they were 79 years old.

He had just arrived from New York to give a lecture at the gallery, and he had brought hundreds of slides, the best work of a half-century, the shots that made it through decades of editors, darkroom disasters and nearly 2,000 assignments for Life Magazine.

One of Life's four original photographers, Eisenstaedt helped form the visual taste of a nation. Startling in 1936-he shot the magazine's second cover; Margaret Bourke-White did the first-he gave us candid pictures of Eistein and Bernard Shaw, Hitler and Churchill, Dietrich and Monroe, and all the hundreds of human icons, who peopled the history of the 30's and 40's and beyond. He is now with the reincarnated Life.

It goes back farther than 1936, of course. Even before he went off to the Western Front in World War I with the German army, he had a little folding camera. In 1927, a button salesman on vacation from Berlin, he captured a Czech tennis player in a poetic study of shadows. And sold the picture for 12 marks.

"I was thrilled when I discovered what you could do with an enlarger. Nobody knew about enlarging then. I could take the arms, or the legs, or the tree branches. I could crop things out. That's when the bug hit me."

Three weeks later, his eye quickened by his studies of light and shadow in Rembrandt and other painters, he photographed an old woman in a railroad station, her head haloed by steam. He still hadn't heard of Steiglitz, but perhaps it was just as well, for he was headed in a different direction, the journalism of pictures.

Working in Europe for an Associated Press photo agency, he developed a new style using natural light and unposed subjects. The word was "candid": frank, open, unbiased.

"We did a lot of conferences: the League of Nations, disarmament talks, monetary discussions at The Hague. It wasn't what you'd call hot news. We worked in tails and black tie. I had reinforced pockets in my tails to carry the big steel cassettes, the glass plates that I'd develop in hotel rooms later. Usually I used a tripod. And I'd take one picture. One picture."

When Life went to a certain sports event recently, he said, it wound up with 273 rolls of film....and used three pictures.

"Ah, Life," he smiled. "An elite group. Before, the editors would always tell you what they wanted. But at Life, you were on your own. I went off to do a series on colleges and universities, and all the editor said was, 'The only thing I don't want is to see blood.'"

He did a photo essay on ethiopia six months before Mussolini invaded it and years before anyone had heard of a photo essay. He photographed the Galapagos Islands while sharks bumped against his small boat. He spent a day hoisted 40 feet in the air taking pictures of a tropical rain forest. Shooting the fearsome gorilla Gargantua, he had to wear a rubber tire around his middle in case he got so absorbed in his work that he leaned within reach of those long hairy fingers.

The gorilla shot was the finale of his lecture last night, after Marilyn Monroe and S.Dillon Ripley.

Marilyn:"She was so frail. She was much thinner than I'd thought. It was in '53, for a Life cover. I had the wrong camera. I forgot which was the color and which was the black-and-white.

Hemmingway: "You had to be careful what you said to him," Eisenstaedt recalled recently. "He had all these terrible cats around." (It is a celebrated story. "What's the matter, don't you like cats?" the author roared. Eisenstaedt, who can't stand cats, gulped as he felt the fragile relationship cracking. He managed to say something about the tripod being jiggled, and Papa was mollified.

Sophia Loren:I've been taking her picture since 1961. A beautiful woman. She still writes to me."

All the mistakes a photographer can make, he has made: taking pictures with the lens cap on, opening the camera to see if it had film in it - and, yup, it did. Once he spent a half hour capturing the essence of a giant cake, two stories high, for the Bicentennial while suspicious guards kept the crowds away and officials champed at the delay. Then he found that he had no film in his camera.

He works with a 35-mm mostly. Has a big Linhof view camera but never uses it. Doesn't own a motor-drive.

"Some people are under the misapprehension that having the most expensive camera and dozens of gadgets is a sure path to good photography," he writes in his latest book, "Eisenstaedt's Guide to Photography" (Viking Press). "It's the eye behind the camera that counts most."

Probably he is best known for his candid portaits of the famous, and by now he is into his third generation of celebrities, from Jackie Kennedy to Henry Kissinger, but in the great Family of man exhibit at the Museim of Modern Art, it was his shot oh some children aping a drum major that people remembered.

He loves nature pictures. "It's my avocation too," he said. "Ninety percent of fit I do for myself. I was sent to India to make pictures of monkeys, and the least thing I took was monkeys."

Of course, when Life turned to color photography, Eisenstaedt did color. He likes color , he insists, this lifelong watcher of light and shadow.

"But when I dream, you know. I dream in black-and-white."