This excellent collection of Callahan's photographs accompanies a national tour of his work. Curator Greenough's (Robert Frank: Moving Out, LJ 10/15/94) decision to arrange the images chronologically works well to illustrate both the themes central to the photographer's aesthetic and his development as an artist. From early experiments using multiple exposures and light painting to the most recent color cityscapes, Callahan has sought to explore photography's potential. He often returned again and again to the same subject in a quest for yet a new way to "see" it via the camera. Now in his eighties, Callahan is a 20th-century master of American photography who places the highest value on the process of self-realization through image-making rather than on any individual photograph or series of photographs. His life's work stands as convincing testimony to this ideal. This retrospective will be a fine addition to public and academic photography collections.