I don't know if this will help but until now I've always used filters to protect my lenses but recently bought a Minolta 75 -300 Big Beer Can with filter online only to find it really poor over 200mm - to cut to the chase it seems that the filter (a recognised make) & the lens were incompatible & once I took the filter off, the lens was as sharp as it should have been.
I suppose the thing to draw from this is "try before you buy" - for the record I have found Marumi filters worked well for me.
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