Seems precise made and inexpencive (US $9.90 on ebay) compare to other rings. http://proactivegr.com/gears/
The gears are designed for ANY LENS diameter from 50 to 93mm!
Separate gear made for each diameter of 1 mm increments!
The gears are made of rigid polyurethane, that stretches a little bit, when you put the gear on.
The correct form of the teeth module 0.8.
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