As photographic technology advanced—cameras became more portable and film more sensitive to light, requiring shorter exposure times—people were no longer required to pose for pictures. In an effort to capture candid images of people in public places, Walker Evans affixed a right angle viewfinder to his camera to make it look as if he was pointing it off to the side rather than directly at his subjects. For his Subway Portraits, he went even further and concealed his camera by painting its shiny chrome parts black and hiding it under his topcoat, with only its lens peeking out between two buttons. He rigged its shutter to a cable release, whose chord snaked down his sleeve and into the palm of his hand, which he kept buried in his pocket. As a result, these portraits show people in unguarded moments.
Additional text from Seeing Through Photographs online course, Coursera, 2016