Most people will acknowledge that the camera can be used as a powerful tool for telling lies. Harder to accept is that the camera, by its very nature, always lies, that it always misrepresents, distorts or manifests the hidden manipulation of its operator.

But it’s a truth ineluctable after spending time in the National Gallery of Art’s fascinating and provocative “I Spy: Photography and the Theater of the Street, 1938-2010.” The exhibition is devoted to six photographers who explored ways to photograph surreptitiously, or without intruding on the drama of their subjects’ private existence. In Walker Evans’s 1938-1941 “Subway Portraits,” the photographer concealed a camera in his coat and captured straight-on images of people sitting opposite him on a train. Decades later, photographer Philip-Lorca diCorcia rigged elaborate photographic traps in New York, concealing synchronized flashes and using a pre-focused telephoto lens to record spontaneous moments of street life with the professional polish of a fashion photographer.

The assumption driving these experiments is simple but problematic: By masking the presence of the photographer, one can get a deeper, more unguarded truth about people. As Evans put it, he wanted to capture people “in naked repose,” with their guard down and “the mask” off. Whether it’s Freudian slips of tongue, unwanted conversations caught on a hot mike or leaked videotape from cameras no one knew were on, we tend to believe the spontaneous self is the honest self. But it’s a quirk of modernity to believe that the social mask is false and that there is some kind of genuine authenticity underneath it.

Study the images Evans made in the subway, and it’s hard to believe that many of his subjects were unaware of his presence. Perhaps they didn’t see the camera, but several seem cognizant of something odd about his presence. It’s one thing to rig a camera to peer out between the buttons of your coat and respond to a shutter cable in your jacket; it’s another to mask your own presence in a highly artificial game. Some of his subjects might be oblivious, but at least a few might be giving him a subtle fish eye or awkwardly avoiding him.

Evans changed the equation in another series made in Bridgeport, Conn., in 1941. Standing on the street, using a larger viewfinder camera, Evans photographed passersby without hiding the camera. The faint, questioning looks one sees in the “Subway” series become a catalogue of annoyance and confrontation, as his Bridgeport subjects look sideways and down to where the camera was positioned at Evans’s waist. The angle of their gaze is the same as it would be if Evans’s fly were unzipped, or if he were engaged in some other waist-level indiscretion. Perhaps there’s humor lurking here: The voyeuristic photographer makes himself a spectacle for others.

Dismemberment

The so-called “observer effect” in physics — that merely observing a system changes it in some way — comes to mind as you travel deeper into these experiments.