One of my favourite articles in the Kodak Encyclopedia of Practical Photography is the one called "Tone control." It explains, using quadrant diagrams, the relationship between the various components of a photographic system: subject, lens, film, paper.
I love quadrant diagrams. They really make it easier to visualize how the lens, the film, and the print all compress and expand the relationships that exist between all the tones in the original scene to the final print.
I wish there was some software out there that could do this kind of diagram. I'd buy it.