Adjusting S4 Zoom

HI

we're adjusting our S4 Zooms, well at least we try.
Some are pretty fine, but on soom S4s it seems to be rather impossible.
In the middle of the spot, it is most time OK, but at the edge of the spot we got problems.
Sometimes there is a "shadow-ring", about 10 centimeters from the edge.
At other S4 we got "eyelashes" at the edge.

 

Any Idea how to get rid of them?!

Thanks

Lukas

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