Depth of Field for Macros (1 Viewer)

As we all know one of the biggest problems with macro photography is the small DOF. As an example shooting a 100mm focal length from 2ft at f8 gives a DOF behind the subject of just 0.19". With longer focal lengths the problem gets even worst - 150mm focal length from 2ft at f8 gives just 0.07" in focus behind the subject. Doubling the f stop will double the in-focus distance.
For anyone who is interested there is a useful on-line calculator HERE

Great link Roy.
BTW, I have an Olympus E-520 plus Zuiko Digital ED 50-200 mm f 2.8 - 3.5 SWD. I ordered the EX-25 extension tube for close-up/macro. The minimum focusing distance for my lens is now around 1.2 meters. Any idea about the minimum focusing distance after adding the extension tube?