Recently i bought a bunch of Rollei round filters. Those filters are special in the regard that they are not classic screw filters, but magnetic filters you clip onto each other.

This does mean though, that you need a Base Ring which actually attaches to the lense by screwing. The filters themselfs do not have to have the same diameter as the lens though, for that purpose Rollei sells seperate Step up rings, which connect a smaller diameter size to a bigger one.