Let me try and answer several questions in one go.
1. Quality - without even resorting to the great Putts who found the Hexanon sharper wide open than the latest Elmar, I would say this:
- in comparison to the Skopar F2.5 which I own, it simply wipes the floor with it, beats it wide open, better bokeh, better color, it's just a vastly better lens.
- in comparison to my ZM SOnnar F1.5, MUCH sharper wide open (no surprise there) though teh Sonnar has other charms.
2. FIlter size: I believe it is 40mm (unles thatw as 41) in any case I had no trouble finding a protective filter for it - it is NOT an exccentric size.
Hope this helps.