Robert Frank, from “The Americans”

Robert Frank’s „The Americans,“ at the Metropolitan Museum of Art, includes Frank’s photograph „Elevator — Miami Beach,“ 1955.

Robert Frank’s disturbed and mournful song-of-the-road portrait of a new homeland is the subject of a 50th-anniversary exhibition now at the Metropolitan Museum of Art.

For some people, a camera is armor. For Mr. Frank, it was an antenna, a feeling and thinking device.