Bernd and Hilla Becher's industrial art

To photographers Bernd and Hilla Becher, the rapidly vanishing industrial architecture of Western Europe and North America were works of art. The German couple's documentary images of transmission towers, gas tanks, blast furnaces and smokestacks – structures that signified the end of an industrial era – are being celebrated in a comprehensive retrospective now at the San Francisco Museum of Modern Art. Lee Cowan offers us a tour.