Sarah Kennel, associate curator, department of photographs, National Gallery of Art, Washington. The extraordinary range and complexity of the photographic process-from the origins of the medium in the 1840s to the advent of digital photography at the end of the 20th century-are explored in a comprehensive exhibition and accompanying guidebook. On the occasion of In the Darkroom: Photographic Processes before the Digital Age, Kennel talks to host Barbara Tempchin about the major technological developments in the 170-year history of photography.