The following videos are very informative in providing a really good explanation how the Depth of Field of works. One very useful tool is the Canon EOS Utility allowing one to control their camera from a desktop computer or laptop. Its an interesting interplay between the focal length of lens, the distance to the subject and the aperture. This webpage may help to give some further information featuring elements such as the circle of confusion and the hyperfocal distance, you will even find a simple DOF calculator at the bottom of the page. The next page on dofmaster.com provides another useful dof calculator and some useful illustrations. You may also enjoy the following DOF equations, providing details on the hyperfocal distance as well as near and far distances of acceptable sharpness.