http://dofmaster.com/dofjs.html is one great tool for understanding that shooting “wide open” doesn’t automatically generate good depth of field (DOF).
DOF has to be balanced; three factors decreases the depth of field:
1. Wider aperture (lesser f-number)
2. Longer focal length.
3. Shorter distance to subject.
Smaller sensor (film/frame) apparently also makes DOF changes – see http://en.wikipedia.org/wiki/Depth_of_field#DOF_vs._format_size for more info on that.
Two stops on the f-number (i.e. multiply by 2) and the depth of field will quadruple (multiply by 2*2).
Double the distance and that also quadruples the DOF. Approximately, and at close distance – where it matters to know about it.
Generally, using wider than f4 at a one meter distance (~ three feet) equals a DOF of a few centimeters (2.54cm = 1″).
I hope you do know that one stop equals multiplying the f-number by 1.414 (squareroot of two). Two stops, multiply by 2…
The same goes for half or double the time on shutter speed.
(Half time = half the light = one stop less)