In 1949 Georgia O'Keeffe chose the National Gallery of Art as the custodian of nearly 1,600 photographs by Alfred Stieglitz, the Key Set as it has become known. With the formation in 1990 of the Gallery's department of photographs under Sarah Greenough, the collection has grown to 14,000 works of art, an assemblage that both charts the development of the medium and reveals the beauty and dynamic versatility of photography over its course of more than...