Depth of Field Simulator
Depth of field is a well-known concept in photography, and generally not quickly internalized.
I believe this is true partly because it is not intuitive and partly because most explanations
rely on the mathematics, which involve at least three variables: f-stop, distance from subject,
and length of lens.
Photographers by nature are mostly visually-oriented, which makes it seem reasonable to present the concept graphically.
So, I wrote a Depth of Field Simulator, a small Java applet that gives immediate, visual feedback showing the effect of various configurations. The user can adjust sliders to manipulate zoom, distance, and aperture, as well as rotate the subject in space.
There is also a stereographic version, if you like that sort of thing.
has a good discussion about depth of field, without a lot of complications:
Paul van Walree has a more in-depth derivation of the formulas,
including treatment of asymmetrical lenses (and other optics topics) www.vanwalree.com/optics/dofderivation.html.
Norman Koren really lays it out in www.normankoren.com/Tutorials/MTF6.html.