What is the theme in On Photography by Susan Sontag?

Simple mechanical documentation is a theme in the book. The earliest forms of photography, whether daguerreotype or calotype, use the same basic process: a substrate of paper or glass is coated with photo-sensitive chemicals and then exposed to reflected light emanating from the photographic subject. Various chemical methods, from the simple to the complex, are then used to develop the photo-sensitive chemicals such that they show a mimetic image of the photographic subject—though usually reduced greatly in scale. Since the earliest photographs of 1839 to the time of the text's publication in 1977, the photographic process hardly changed in conception, though obviously constant refinement in equipment, films, and papers was pursued with enormous success.