A century of American photography in one afternoon: this rare exhibition will be greater than the sum of its parts

Left photo: A shirtless construction worker grins as he clings with easy confidence to a giant iron hook and pulley high above the Empire State Building construction site.Right photo: A woman leans exhausted against a bearded man in the front seat of a worn car, while two young children stare directly and blankly into the camera from the back.

There are exhibitions where you walk around admiring individual photographs, and there are exhibitions that change how you think about photography itself. Portrait of a City: A Century of American Photography, opening at Dulwich Picture Gallery in London on 28 July, very much fits into the second box.

With more than 100 works by 38 photographers, it traces American urban life from the early 1900s to the close of the 20th century. What makes it genuinely valuable isn't just the quality of the individual images, though that's considerable. It's the sweep: the chance to watch photography figure out what it's for, in real time, across a century of history.