Wirelessly posted (BlackBerry9000/4.6.0.167 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102 UP.Link/6.3.0.0.0)

I can't tell you those minimum distances, though there is a mathematical formula to determine it. However if your lens focal lengths are a constant, when you change the focal extension from the film plane by a constant distance, the ratio of minimum focal distances between one lens and another will remain constant as well. If you have focal distance of, say, 1 with the 100mm and a min focal dist of 2 with the 200mm lens then your min focal distance with the 100 and a 50mm ext tube will be 1/2 the min focal dist of the 200 with the tube.