Five galleries on P Street which often hang paintings, prints or drawings have now simultaneously opened photo shows instead. Art photo exhibitions, once relatively rare, are novelties no longer - and the scores of them now teach us to discriminate between the good and the bad.

We have learned to tell the difference between seers and mechancis, artists and technicians. Even well made photographs, we've learned, can be wooden and dull. Photographs have not grown worse, it is our eyes that have improved. In works that would have once seemed fresh, believable and real we now recognize the heavy hand of conventio hand cliche.

Gallery Rebecca Cooper, 2130 St. NW, for instance, is showing pictures of Plains, Ga., by Washington's Susan Irwin. Some of them are lovely, and the town is well portrayed, but the good looks of her pictures and their sociological accuracy aren't enough.

The weathered church in center frame, the black sharecroppers, their tin-roofed shacks, the gracious columned mansion - almost every picture seen here is a picture seen before. Irwin's reportorial mood of affectionate detachment is equally familiar. Perhaps it is too late for a northerner to show us the contrasts of the South. Here precedents are ruinous. The ghost of Walker Evens tramples on this show.

Frank DiPerna's photographs of Mexico, now on view at Diane Brown's, 2028 P St. NW, are also much diminished by the conventions they obey. Some of them are beautiful, but the dead burro by the roadside is an image that no longer shocks; neither does the contrast between Old and New - the new car in an ancient street, the electrical transformers sprouting from a shrine. Like dozens of photographers who have been to allen lands before him, DiPerna selects surreal images - the bus upon the roof, the giant chicken on the wall, the baseball player carved of stone standing in the graveyard - but the eerie strangeness of these sights is itself cliche.

DiPerna, who teaches at the Corcoran, is best when least portentous. His gifts are undeniable. When he avoids The Message and shows us sunlight on white walls, a dog and nets at Ajijic or the foliage of jungles those gifts are clearly seen.

Another well-known photographic mood, supercilious reportage, saturates the photographs of Linda Rich at the Intuitiveeye Gallery, 641 Indiana Ave. NW. Her subjects are the roadside wedding chapels of Nevada, Cowe Haren in the Poconos (a resort with heart-shaped bath tubs), and the newlyweds they serve. Rich shows unhustlers and misfits and innocents as well. Except for one that's full of life - a young bride in a tree - their mood is snide and stale.

The difference between good and dull, between artifice and art, is seen with special clarity at 2014 P St. NW, where Allen Appel and Rom Stark are showing photographs of nudes.

Appel's photographs of Megan (exhibited at the Studio) are beautiful, passionate, unexpected.The subject is his wife. She crouches, floats, glares, seeming to be a dozen women. The study of the nude is among the oldest of art's old conventions but these pictures, technically superb, have about them something fresh. None seems merely posed. One feels her body's weight, the stretching of her sinews, the movements of the sun, alls seen in a mood of honesty and intimacy.

Ron Stark's nudes, in conlrast, seem frozen, formal, dull. He has many admirers, but I am not among them. For years he specialized in carefully conceived shots of fish and fruits and vegetables. His models, though beautiful, resemble statues more than women. He might as well have photographed eggs or oysters, so void are they of life.

Emmest Gowin, whose photographs are on display at the Washington Gallery of Photography, 216 7th St. SE, is another fine photographer whose highly personal pictures manage to transcend conventions. In many of the best of them his subject is his wife. His pictures make one think more of transcedent snapshots than of formal studies of the nude.

Something ineffable, some breath of beauty and of life touches the most casual of Gowin's images. Children play on grass, a glance is exchanged, an old woman sits upon a bed - nothing special happens, but still these pictures have a stabbing power.

The invitation for Firooz Zahedi's photo show at Bloomingdale's was so heavily armed for impact it almost jumped out of the envelope. The single photo reproduced shows Liz Taylor and Abe Lincoln. The blurb inside ("If I could, by magic, turn top photographers into cameras . . . I would like Firooz to be one of them") is by Andy Warhol. And then there is Zahedi's surname. The Iranian ambassador, Ardeshir Zahedi, is the artist's cousin.

Firooz Zahedi is a 27-year-old artist who graduated from the Corcoran in commercial art in 1976. Though he is not selling clothes, his pictures resemble those of the fashion magazines. They are striking and exotic. The Liz Lincoln shot is a winner, but its is the best of them. Others in his show have been damaged, somehow cheapened, by conventional hand coloring.

The Foundry Gallery, 2121 P St., is showing Joe Cameron's grouped photographs. Though one of the most original and gifted of Washington's photographers, his show is somehow injured by its quicky presentation. The tiny prints are grouped as if to illustrate some private abstract narrative. Cameron can pour mystery and emptiness into simple shots of dogs, a chair or a beach, but his titles seem unnecessary and his peculiar display seems to keep the viwer away from the show. It closes Feb. 5.