Congrats on the lens! I would test the filter as previously suggested in this thread (by Brant, IIRC without going back a page). Promaster does not make filters (or anything else). They sell re-branded products, and in the case of filters, the OEM is Hoya. Hoya makes several lines of filters, sone of which are optically good (as good as B+W, but reportedly harder to clean). But, Hoya also makes some lines that are of pretty poor quality. Which line is rebranded as Promaster HGX? I don