I acquired some drop in type filters. I couple will fit my Leica Summicron-R 50mm lens with a screw in filter holder just fine. The others are too large for the Summicron. I think that they would fit on my Leica Elmarit-R 28mm with the same time of screw in filter holder. The problem is I am not sure what filter size the lens is. How do you measure filter size? Using a small ruler it seems to be either 48 or 49mm. Searching online I only see reference to the filter size being 55mm. It is clearly not 55mm. I have a 55mm filter holder that is much to large for the lens. Am I measuring the wrong thing? Does anyone have a good website for Leica R lens information? Thanks.