When, in 1907, Alfred Stieglitz took a simple picture of passengers on a ship bound for Europe, he could not have known that The Steerage, as it was soon called, would become a modernist icon and, from today's vantage, arguably the most famous photograph made by an American photographer. In complementary essays, a photo historian and a photographer reassess this important picture, rediscovering the complex social and aesthetic ideas that informed it and explaining how over the years it has achieved its status as a masterpiece. What aspects of Stieglitz's ideas and sometimes-murky...