I have recently acquired a Hasselblad kit which includes the 60mm f/5.6 C lens. It is slow, especially for interior viewing (although since I do 4x5 large format with many f/5.6 and slower lenses I'm not as disturbed by this as some might be), but nice and sharp. The filter thread is "63" I can screw in a 67mm filter part way, but I'm afraid of doing damage to the lens or filter threads if I tighten the filter. (I have alot of 67mm filters for large format.) Does anyone know if there is some kind of adapter that properly converts the "63" size to 67mm for regular fllters. Also why is it designated 63?}