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 urge anybody in proximity of a public library to look for that article (and read the whole encyclopedia as well!). It is a great complement to PE's article, and will help in providing you with a good vulgarized background on image tone management.

Of course, Kodak's article is just about explaining how things work. I think PE will go to the point at which we will understand also how to design them...