51mm Filters

I have a very nice Cosinon 135mm manual focus lens which is I use for macro when used with extension tubes. It has 51mm thread size and I cannot get a protective filter anywhere in this size. I have also looked for a step up ring but no luck there either!
Does anyone know if this odd size is still available?
I would be interested to know if anyone else has come across a lens using this sized filter.

Are you absolutely sure it is 51mm? This tends to be a push-on size for lens caps. It is almost unheard of for a manual focus 135mm lens to be anything other than a 49mm or 52mm fuilter thread. 51mm is sometimes the outside diameter and the inside filter thread would be 49mm.

If it really is 51mm (and I agree with John on this, unlikely) about the only solution would be a custom one made by SRB but it would be £'s

Kim

49mm is too small and 52mm too big! I could try clingfilm????????

Cheap soft focus filter.

My only other thought is that it might be a Series thread, probably Series VII.

Kim