Who sells 10cm tubes
?
I live in the US. You can't expect me to keep straight those units from the French Revolution. I don't even approve of chopping off people's heads.
I looked for my notes from when I was trying different tubes out, but did not find them for the 100mm. Interestingly, I did find them for the 24–105mm non-L zoom. I got 1:1 with a 36mm tube and the lens at 32mm. At 105mm, I got 1:1.89, or 0.53x.
I did some rough tests with and without the tubes. I just eyeballed alignment and focus, and probably not carefully enough. I shot pictures of a ruler (in inches, of course) to calculate magnification. These are 100% crops I believe from pictures taken with and without tubes on the 100mm non-L macro lens. The three tubes together add up to 68mm (6.8cm, 2.68 in., 0.149 cubits), so that would be the maximum for what I must have used (so not really that far off 10cm). So first 1x and then 2x (a bit overexposed):