Official Washington tonight will gather at the Corcoran to grant to art photography a kind of benediction.

Around 250 guests from the White House and the Congress, the diplomatic corps, corporation board rooms and the posher streets of Georgetown will attend the seated dinner. The ladies in their gowns, the men in their tuxedos, will - by their very presence among those small, machine-made pictures - be sending us a message: The collecting of photography is socially acceptable; the Establishment approves of photography-as-art.

The images on view will not disappoint them. This is the finest show of photographs Washington has seen. They are old, new, costly, cheap, subtle, shocking - but there is one thing that they all reflect: Sam Wagstaff's private taste and educated eye.

The Establishment moves slowly; it did not jump, as Wagstaff did, from the safety of fine paintings into photography's stormy market. But the Establishment trusts Wagstaff. He is one of their own.

He is a Wagstaff of the New York Wagstaffs, Yale '44. He is descended from collectors; his father purchased prints, and his uncle, David, built a private library in Tuxedo Park. Samuel J. Wagstaff Jr., 55, is aristocratic, wealthy, and he looks the part. There crackles round him an air of dissipation, but the Establishment is willing to overlook such static. He has art credentials few photography collectors can match.

Photography is not his first art career, but his third.

Wagstaff, after Yale and a stint at NYU, was one of the first two scholars to be given a David Finly fellowship by the National Gallery of Art. With the money given him he traveled Europe as a gentleman, looking at old masters, doing research at I Tatti, and then settling in Paris to work on Paul Gauguin. Wagstaff might have stayed in the placid fields of traditional art scholarship, but instead he chose the turbulence of '60s modern art.

As a curator in Hartford, and later in Detroit, he showed minimal sculpture, earthworks, pop art, all the newest things. And while he was doing so, he put his money where his taste was. He bought "The Deep" by Jackson Pollock ("that one didn't cost me merely pennies"), which is now the only Pollock in the Place Beaubourg. He bought Warhols, Agnes Martins, Oldenburgs and Lichtensteins. Then he dropped out for a year.

"To be frank about it," says Wagstaff, "I got cranky about the '60s. I got to Tony Smith, to Agnes Martin and Walter de Maria, but then my interest lagged. Nothing took me farther, until I found photography and again turned on."

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Photography collectors tend to fall into two groups. Some prefer to specialize, buying only, say, 19th-century French material, or Western things, or portraits. Wagstaff is not one of them. He buys anything he likes, postcards, for example, or Victorian stereoscopic views. He buys landscapes, still lifes, portraits, and oddities. He bid $150,000 on an album full of photographs by Julia Margaret Cameron, but the British would not let his album leave their country and so bought it back. He spent $9,000 on the Thomas Eakins now hanging at the Corcoran. He owns three photographs by Degas, who was an even finer painter. "Sometimes you get lucky. My Degas works were freebies," Wagstaff says.

What makes his show so splendid is that it greets the viewer in a thousand different ways. Interested in animals? Here are dogs, goats and hippos. Historical portraiture? Here are Lincoln, Greta Garbo and Carlvle. Would you like to see the little girl Lewis Caroll ogled, or the landscape of the West, or 19th-century yachts, a syhilitic's sores, an authentic cowboy beside an authentic chuckwagon, or what happens to an apple when a bullet passes through it? It is all in his show.

The catalogue that accompanies the show is not a catalogue at all, but a book of images. They follow no chronology. One is most beautiful than another.

It has a one sentence text: "This book is about the pleasure of looking and the pleasure of seeing, like watching people dancing through an open window. They seem a little made at first, until you realize they hear the song that you are watching."