Helen Levitt's photographs in black-and-white and color, now on exhibit at the Corcoran Gallery of Art, are delicate, humane. They are also boring, narrowed by one convention, one too-familiar theme. All the pictures in it fit into a single cubbyhole -- one already jammed with works by other artists -- labeled "scenes of city streets."

Levitt is a New Yorker, a good and good-hearted artist who has been working the same sidewalks now for more than 40 years. When she first showed her pictures, they seemed strikingly original, it is not her fault that their originality has long since drained away.

It is not her fault that a thousand other photographers have walked on the same sidewalks and shown us the same subjects: children playing hide-and-seek, old folks on the stoops, street signs and graffiti.

Atget made such photographs on the streets of Paris a century ago. Brassai, Cartier-Bresson, Ben Shahn, Walker Evans (Levitt's friend and mentor), Robert Frank, Bruce Davidson -- all were street photographers, and the list goes on and on.

Most painters, too, are specialists -- some paint portraits, some do still-lifes, some concentrate on stripes -- but painters get away with it. Something in their handmade art, some quality of color, handwriting or brushstroke, permits them to work freely in the prison of convention. It is tougher for photographers.

Levitt is no cynic, not does she idealize. But the mood her photos cast -- their sense of drift and dream, of comradeship and loneliness -- is too familiar to be interesting. Only when the viewer screens out Levitt's subjects, and shoos away the precedents that her pictures summon, do her subtle photographs begin to do their work.

They are beautifully composed, for mally impressive. One work on display, of two men by a newsstand, is a brilliant lesson in how to make a photograph blossom at the edge. Two groups of Levitt's pictures -- one dating from the '40s, the other works in color, from the '60s and the '70s -- have been chosen for this show. Corcoran curator Jane Livingston feels that Levitt's color prints suggest "a kind of photographic art which can be unshamedly exciting as chromatic exercise," but I wouldn't go that far. Often Levitt's colors seem a little wrong, her brick reds are a bit too red, her greens a bit too green. The Polaroid Corporation and the National Endowment for the Arts helped pay for the Levitt show, which will remain on view through March 23.

"Painterly Photographs: Contemporary Handworked Images," the eight-artist show on view at the W.P.A., 1227 G. St. NW., is in some ways an attack on the most restrictive of photography's conventions. The pictures on display are the opposite of "straight."

The artists represented (all live in or near Buffalo, N.Y.) do not merely aim and shoot. Instead, they cut up pictures, draw on them and color them, scratch them and distort them. The viewer has the feeling the photographers all wish they could paint.

The weakest works on view, by Roberta Mages, Ann Rosen and Ellen Carey, are so self-indulgent they make the viewer cringe. Carey and Rosen scribble on their images. Mages distorts hers with needless overlappings and superimpositions: they seem only muddy and illegible. Far more pleasing and original are the works of Tyrone Georgiou and Biff Henrich. They run away with this show.

What saves their art is wit. Henrich begins with arbitrary images -- of a bit of wall or window, of people hanging around, doing nothing special -- and then adds to his pictures Xs, lines and arrows which threaten to explain what makes no sense at all. An arc appears to hinge a wrist, an arrow calls attention to the corner of a window, a drawn square frames a coffee cup. Henrich's drawn-on photos are half confusing and half clear.

Georgiou's pun-filled "Polaroid Constructions" are made of SX-70 prints, glue and cut-up cardboard. They play delightful games with traditional illusionism, with two and three dimensions, with architecture, architectural models and the constructivist compositions of the early 20th-century Russians -- and those of Frank Stella. They are sharp and clean and funny; they look fine on the wall. Anthony Bannon, a critic who writes for the Buffalo Evening News, organized the show. It closes on March 1.