From fuzzy images of soldiers pouring from the trenches in World War I to our first views of planet earth suspended over the moon's barren landscape, the 20th century has been defined through the camera's lens. To recall the century is to open a photo gallery in the mind, where the most famous images of our times reside: the joy of a soldier kissing a nurse to celebrate the end of World War II, the horror of a President assassinated in the broad daylight of a Dallas afternoon, the triumph of five gritty Marines hoisting the American flag atop Iwo Jima.