The camera doesn't lie. And yet, when Edward Weston photographed a green pepper, it looked like a naked woman.

Weston was one of the great pioneers of photography in America, ranked along with Stieglitz and Steichen for transforming the medium into an art form. In photographs that need no signature, Weston isolated and focused on what he called "the thing itself" -- whether a shiny, plump and twisted pepper, a gnarled cypress, a woman's breast or the monumental runnels in a desert canyon.

His often-astounding photographs are on view at the National Museum of American Art in "Supreme Instants: The Photography of Edward Weston." His work itself was a supreme instant -- of mastery of the craft -- and the show is a visual feast of texture and light, spiced with amusing juxtapositions. A Mexican toilet bowl looks just like the white shell in the photo hanging beside it, and a dead pelican remarkably similar to a wrecked car. Weston's vision was flexible, and he had no compunctions about disorienting the viewer as to scale.

Things are never as they seem. This is equally true of the photographer himself, seen through the long lens of the retrospective.

Weston was the spiritual leader of Group f/64, which was founded in the San Francisco Bay area in 1932 and included such budding "pure photographers" as Ansel Adams and Imogen Cunningham. The group took its name from the number of the smallest aperture setting common on view camera lenses -- one that produces great depth of field, with both foreground and background in focus. The name underlined one of the group's doctrines: an insistence on sharpness of focus, down to the smallest detail.

But Weston had started out on the other side of the street -- as a dreaded pictorialist. On display here are soft-focus portraits in a mistily romantic style, a` la Gertrude Kasebier -- sentimental studies of friends pensive by a studio window, on textured platinum paper. He likely chose this warm-toned paper because it made the photos look like artists' prints; by contrast, the f/64 group developed a view of photography that emphasized its distinctiveness from any other medium.

The first entry is of a humble tree and path in a Chicago park: It is "Spring," signed "Weston '03," his oldest extant print. When he composed this promising landscape, he was 17, a high school dropout who had been taking pictures for about a year with a box camera his father gave him.

Even his pictorialist pictures are different, individualistic. Rather than emulating 19th-century landscape painters and portraitists, he looked to the cubists and other modern artists in his compression of space. In one early photo, a man and ladder vie for a triangular area. Weston controls the space.

"Weston is taking a modernist sensibility and using a photographer's eye and synthesizing that into a visual image that has everything to do with how we look and why we look at things," says Merry Foresta, the museum's associate curator in graphic arts.

He uses shadows -- things that amateur photographers tend to overlook -- as part of the total composition. A shadow cast by a vase of flowers overwhelms a woman with a fan. A shaft of light in an attic illuminates just a man's face, the rest of the photo is cast in angular shades of darkness.

Opening a studio in Glendale, Calif., in 1911, Weston worked as a portraitist using natural light. There, with the first of his three wives he had four sons (including Brett, who would become a famous photographer in his own right). But in 1923 he moved to Mexico with his lover, taking his oldest son, Chandler, with them. He shared the urge for expatriation with many American writers and artists of his generation, and Mexico City hummed with artistic activity.

"What was going on in Mexico City in that period is very under-studied," says Foresta. "The whole Mexican muralist movement -- and theater people, writers, poets, painters who all knew each other -- it combined to make a very fertile society. He takes that all in, and when he goes back to California, he starts isolating his subjects."

Or perhaps the stark light in Mexico heightened his sensibility. After three years of local genre scenes, portraits and nude series there, he returned to California, where he began to take an object, light it and boldly make it into something different: the whirling roadways in an onion sliced in half. The unconquerable ridges of a cabbage leaf. The lolling tongue of an orchid. A suggestive winter squash. Suddenly, vegetables were scandalous. By then, Weston had switched to gelatin silver paper, with sharper images and colder tones -- which moved him even further from the pictorial tradition.

Weston is known for his edict on composing through the lens. For his first New York exhibition at the Delphic Studio in 1930, he revealed the rationale for his method. It's a message to any photographer who has ever burned in, dodged out, enlarged, cropped or otherwise manipulated a print:

"This is the approach," said Weston. "You must pre-vision and feel, before exposure, the finished print -- complete in all forms, in every detail -- when focusing ... Then the shutter's release fixes for all time this image, this conception, never to be changed by afterthought, by subsequent manipulation. The creative force is released coincident with the shutter's release."

He shot them as he saw them in the ground glass of his 8-by-10-inch camera, propped on a tripod. Hating enlargements, he made contact prints from the enormous negatives.

What fun it is for us to read in the show's catalogue that this pre-visualization was just another rule to be broken. Historian Beaumont Newhall, a curator of the show, writes that he and his wife, Weston biographer Nancy Newhall, went to Point Lobos, Calif., in the summer of 1940 to take pictures with the master. Weston clambered over the cliffs with that huge camera; Newhall decided that his pictures were looking too Weston-like, so he photographed Weston's house instead. Then the master let Newhall use his darkroom.

"When I showed him my prints," writes Newhall, "he asked me, 'Why didn't you dodge them?' This darkroom technique, by which the light is manually controlled, I thought to be a violation of the strict 'no hands' doctrine of pure photography, and to be avoided. Edward, with a chuckle, took me into his darkroom and showed me how he dodged."

It was at Point Lobos, a wildlife preserve on a peninsula jutting into the Pacific three miles south of his home in Carmel, that Weston produced perhaps his best work -- moody combinations of convoluted cypress and rocks, sculpted by water and time into abstract designs. What Weston called his "open landscapes" always verged on the abstract, whereas Ansel Adams nearly always told us where we were.

But they were of the same mind otherwise: "I agree with you," wrote Weston to Adams in December 1934, "that there is just as much 'Social significance in a rock' as in 'a line of unemployed.' All depends on the seeing ... "

Weston kept extensive journals, writing well about why he took a certain photo; his lack of formal education never seemed to stand in his way. He wrote, in one of a number of undated personal notes displayed here: "As a creative medium, black and white photography has, at the start, an advantage over color in that it is already a step removed from a factual rendition of the scene."

But from this comes yet another contradiction in the career of Edward Weston. Late in life, before Parkinson's disease forced him to put down his camera completely, he took color photos of his beloved Point Lobos. Kodak had sent him some film to try out, or it might not have occurred to him.

Weston was apparently pleased, and the catalogue for this show notes his comment: "I decided I like color." And in 1953 -- he died in 1958 -- he wrote in Modern Photography magazine: "You find a few subjects that can be expressed in either color or black-and-white. But you find more that can be said only through one of them."

This was more true than he realized, for the mystery and drama of Point Lobos is lost in color. Weston's control of texture, light and form worked best in black and white.

"Supreme Instants: The Photography of Edward Weston" will be at the National Museum of American Art through January. The Center for Creative Photography at the University of Arizona -- which is the custodian of the Weston archive -- organized the 237-photograph show, and the BankAmerica Foundation funded it.