I use a Canon 10D, and in order to calculate DOF, I use an application called DOF Master ( http://dfleming.ameranet.com/
). The trick to using this application is knowing an accurate value for the "Circle of Confusion" (for mine, I use a value of 0.008). This value is driven by the resolution of your camera's sensor. I found a suitable value for my camera by searching the forums on http://www.dpreview.com/
. There is a forum for each major camera body, so search on the term "circle of confusion" or "coc". The DOFMaster site also has articles on DOF and CoC values for many digital cameras.
Once you have this information, the DOFMaster program allows you define focal lengths for various lenses. There is also a program that lets you produce charts.
This is a great tool, and I use it all the time to determine hyperfocal distances when doing wide angle shots outdoors.
I hope this helps.