Originally posted by Blue You are changing the distance. This is straight up algebra. For a 35mm lens to have the same DOF as a 105mm lens at the same aperture, the distance has to change.
Of course - that was the point of my comment in support of Miserere. To get the SAME SUBJECT SIZE with different focal length lenses, you have to use DIFFERENT DISTANCES. As Miserere said, you have to shoot at twice the distance in order to take a macro picture of a given subject with a 100mm lens vs. a 50mm lens. The change in DOF due to the longer distance cancels out the change due to the longer focal length. The only variable left is aperture.
To show an example from the DOF calculator you linked to:
Pentax K20D, aperture f/8:
100mm lens at 10 inches: DOF = 0.05 inches
50mm at 5 inches: DOF = 0.05 inches
The reason macro photography works this way is because you're typically shooting a particular subject and you position the camera so as to get the subject to fill the frame. This is unlike, say, landscape photography where you shoot from the same position regardless of focal length.