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Interesting, did you just put some tape on the mount, or something more elaborate?
That is precisely what I did. I cut some rings out of clear packing tape (smooth, even thickness, no stretch) with a circle cutter (works like a drafting compass). Instead of applying the entire ring to the surface of the mount, I cut it up into segments in order to modify its tightness. Shorter segments = less tape overall = looser; longer segments = more tape = tighter. Of course, if you had packing tape of varying thicknesses, that would work even better in dialing out just the right amount of play.
On a related note, I'm having fun with the SB, and currently doing a comparison at the ~28mm perspective using the Contax CZ Distagon 2,8/28 vs. Sigma 19mm vs. SEL1018@18mm in a natural shooting setting, just a scene at urban tourist spot (Fisherman's Wharf). Haven't decided how I will post the comparison shots, nor all the conclusions that can be derived from it.
But some impressions I have is that 1) 18mm is indeed the SEL1018's worst FL, and that 2) the Sigma EX DN primes are amazing even irrespective of their bargain price. But the 28mm Distagon w/SB 3) can exceed the Sigma 19's center apparent resolution; 4) is much worse in the corners, but about as good as the SEL1018@18mm; 5) suffers from longitudinal CA, but that is mostly remedied by LR4.
Instead of comparing just across f-stops, all comparisons were done between roughly-equivalent f-stops (i.e. same DoF, which means about the same exposure time).
Edit: backtrack on corner performance between Sigma 19 vs SEL1018. I recall an old test I did with my large bookshelf at home, and coming to the opposite conclusion in favor of the SEL1018. Perhaps the elsewhere-discussed field curvature of the Sigma 19 helped in the particular scene I used yesterday.