And I rushed home just to see how they perform in macro photography. Oh my god... it is well worth the money spent. I've spent hours and hours of research on and off the board to see how I can take better close up macro shots shy of spending a large sum of money on macro lenses.
I have a newfound respect for these extension tubes. I used my Canon 18-55 F/2.5-5.6 IS lens to take some test shots and I was very pleasantly surprised by the outcome.
I took 5 shots. In all the shots I moved the camera back and forth to find the minimum focus distance.
#1. Lens at 18 mm
#2. Lens at 55mm
#3. 36mm tube with the lens at 55mm
#4. 36mm + 20mm tube with the lens at 55mm
#5. 36mm + 20mm + 12mm with the lens at 55 mm 
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Here are the tubes. They have the hook ups for the AF mechanism, but I ended up having to turn off AF since the AF had a tendency to go crazy.
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