Can anyone give me an idea on what the focusing distance is on a 50mm reversed on the 100mm f/2.8 in front of a full stack (68mm) of tubes? 
Reason I ask is since I can't swing the budget to buy a decent set of used bellows just yet I'm thinking of in the meantime I'll just get me a coupler to reverse my 50mm f/1.4 on a 100mm both of which I already have (along with a set of kenko tubes) so I can play around with something beyond 2:1 which I can reach with the 100mm and tubes. Is the IQ from such a setup worth it? I'm not expecting it to be as wieldy as a MPE, but we're already headed into winter here so I'm not going to be using it to chase bugs 
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I figure for $5 for a coupler ring can't hurt to try, I want to make sure there is enough distance though so I can fashion a lens hood of sorts out of some spare lens caps so I don't goof up the rear element on my second favorite lens.
