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Last week I was manually focusing my camera lens (300mm f/2.8) with next to zero luck.
I fitted up a home built stepper system based on the SGL automation teams designs, and use a 5:1 gearing ratio and despite the theoretical 17.4um cfz I am able to achieve focus quite easily and repeatably. I do not however have to contend with focusing for different filters at this stage so it's hard to say what impact that will have and I'm yet to do any serious testing with regards to temperature (other than knowing that I have to refocus every 10 minutes or so because focus shifts noticeably every 1 degree of temperature change.
Prior to gearing it 5:1 I had it geared 2:1 using a drive when instead of a belt drive and found I had nowhere near enough resolution in the step sizes to find a 17um focus point.
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