Given the interest in this topic I thought I'd do a quick and dirty demo of the sort of magnifications that these can achieve. For this demo, I used the KM 18-70 3.5-5.6. This is the budget kit . It's NOT a macro , so you should achieve similar results without much effort.
Fiirstly, the setup. Pretty self explanatory - see the picture. I tested all the shots at the same f9, 100 using wireless flash:
I took the next photo at the minimum focal distance for the kit length. THis is the highest "macro" magnification it's capable of (remember - this is NOT a macro ):
I then took the same shot with a 12mm extension tube:
Next, with the 20mm extension tube:
Next, the 36mm extension tube:
Next, all 3 tubes stacked (total length 12+20+36=68mm):
I have not sharpened nor altered these shots in any way - straight from the camera, converted from to jpg. If you want to get into macro, these are a good way to go. However, bear in mind that you will lose infinity focus. I've used these on several of my , as well as my 105mm macro with good effect. Very versatile. Mine are Kenko brand, and have all the AF pins, so they're fully automatic.