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Actually, there is a point to this. One of the best aspects of OM cameras is the built-in focus bracketing. However, I'm finding that as I push the magnification more and more (here the OM 90mm macro lens plus Raynox DCR250 stacked with a Raynox DCR 150, giving a total of ~5x magnification on the sensor), the quality of my images is at best hit and miss - I'm simply asking the OM software to do too much with hand held stacks. So I sold my old full size tripod which I never use and more and bought a cheap(ish) tabletop tripod.
Problem solved.
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Answer: Chia seeds, ~1.5mm length.