I just got one of my daughters a Pentax ZX-M camera for a photography course that begins next month. At the suggestion of TPF members, I also got her a used manual focus 50mm f/1.7. This is a great lens and I got it in LN condition. The option presents itself, however, to adapt a good filter to this manual lens. Any suggestions regarding which is the recomended filters among the types and size(s) that would fits a manual Pentax 50mm?