The idea of making abstractions with a click of the shutter sounds kind of contradictory, but Aaron Siskind has been at it -- with skill and integrity -- for more than 40 years.

The dean of American abstract photography is 81 now. Siskind is a little guy -- "5-foot-2, maybe." His laugh is big and easy. He's urban and he's smart. When he was 12 years old, skinny Aaron Siskind was giving socialistic speeches to street crowds in Manhattan. In the '30s he already knew Barney Newman, Adolph Gottlieb and the young Mark Rothko. No photographs hang more comfortably beside their abstract expressionist paintings than those that he has made.

Two exhibits of his work are now on view in Washington. The one at the National Museum of American Art is called "Homage to Franz Kline." A survey of his art at the Martin Gallery, 3243 P St. NW, went on view last night.

If you've flipped the pages of a history of photography book, you've seen Siskind's elegant, ambiguous, almost Oriental pictures. They've been greatly influential. He might make a close-up of a strip of peeling tape, or a wall scrawled with graffiti; he might photograph cooled lava, piled lumber, sand shapes at the tide line or a wave-washed seaside stone.

Sometimes, but not always, the eye that scans a Siskind is offered clues to subject and to scale. Certain half-familiar images -- a nail head, a brushstroke -- or the words and signs of typical graffiti -- "Jose'," "Raul," a dollar-sign, an arrow through a heart -- remind us that we're looking at markings on a wall.

But often it's not easy to know exactly what one's seeing. Already in the '40s, Siskind was making photographs that look like Pollocks or Klines. "First and emphatically," writes Siskind, "I accept the flat plane of the picture surface." The early field painters of the postwar New York School often used such words. Siskind's pictures -- with their loyalty to flatness, and their distant and ambiguous attitude towards content -- are just about as close to abstract expressionist painting as photography gets.

Except they're not really expressionist. And they're not really abstract.

His peeling paint and rusting signs, his scrawls and sun-bleached walls, reiterate an old, not-at-all-abstract, almost literary theme, a theme that might be called "the poignance of decay." It has both romantic precedents and memento mori overtones. It shows up in the bruised fruit of 19th-century still lifes, in the Wyeths' weathered barns, and in documentary photographs from Walker Evans' to Bill Christenberry's.

Siskind's expressionism, too, is kind of inside-out. A dozen pictures in his "Homage to Franz Kline" show a wall in Jalapa, Mexico, a wall that has been covered with scrawled-then-painted-out politicians' slogans. Those photographs of canceled letters, of crossed-out Ns and Os, do look a bit like Klines -- until one considers just how they were made.

The two men were "natural friends," says Siskind. "I was single when I met him. We shared an attitude to life -- to art and life and women."

But Kline was an "action painter." His brush was big, his strokes were bold, and when he made his gestures, he used his whole arm. In contrast, Siskind's pictures tend to show the small. They all suggest the tripod. No suddenness, no violence, but something still and poised, a meditative quiet, an almost Japanese exquisiteness, is apparent in his art.

Franz Kline splashed his thick black paint. Siskind's printing is immaculate, so immaculate, in fact, that his photos at the National Museum are shown naked, without glass. "I can't work messy," says Siskind. "I got the mess inside. I want the outside orderly. Otherwise, I get all confused."

Siskind, then a grade-school teacher, began taking pictures in 1930 while on his honeymoon in Bermuda. He had already tried his hand at poetry and music -- "and failed at both," he says. He began working with the Photo League, "a communist front organization." He made photographic essays of the buildings in Bucks County, of Harlem and Martha's Vineyard. He worked later at Black Mountain College (as did Josef Albers, Robert Rauschenberg, John Cage), and then taught at Moholy-Nagy's Bauhaus-influenced Institute of Design in Chicago.

"But I always disliked the Bauhaus idea," says Siskind. "They believed in experiment, in objectivity, in figuring things out. I didn't. I liked the more direct. Action and reason are separate things. You can always pile reason on both sides of the fence. But the decision -- that's not reason. That's something else."

In Chicago, Siskind taught with Mies van der Rohe. ("You'd think from his structures that he was pin-striped, organized, strict. Not at all. He was genial and gentle.") Siskind now teaches at the Rhode Island School of Design.

Sometimes Siskind's images seem to ask the viewer to take a sort of Rorschach test, to find the image hidden in a near-abstract design. That shape of cooled black lava seems a buried doll, that crossed-out X a dollar sign. A set of three photographs at Martin's, all taken on the beach between 1947 and 1953, suggest birds and human bodies. There is poetry aplenty in Siskind's gentle art. His museum show runs through Feb. 24. His exhibit at Martin's closes Jan. 2.