That’s wild. Both of My 3-16 stac scopes is very clear. I even sent in the 4-20 to be checked out and Alan said it was one of the best scopes they had put on their tester. It just wasn’t to me.I also have an STac 4x20. And I have an Siii 6x24x50 and a STac 3x16x42. To my eyes (read that twice - to my eyes) - the 4x20 seems dull, not vivid. The Siii is vivid and I like it. I haven't used the 3x16 much, but so far it seems as good as the Siii. I have never used all 3 side by side for comparison.
It is my understanding that the Siii is assembled in Japan. STac ffp is assembled in Japan and has a zero stop. STac sfp has no zero stop and is assembled in the Philippines.