25.5 doesn't fit. I have several filters that size and they are too large to screw in. This is an early lens so I guess a non standard size isn't a huge surprise, but I hope I can find something that will screw in.
Only the 27mm and 42mm slip-on filters were catalogued in the US prior to the war. None of the Contax lenses used screw-in filters prior to the Contaflex/Contax II era. Filters for the Tessar 5cm lenses were prior to the war were always catalogued as push on style.
Thanks for your reply--I do own a 5 cm f/2 black and nickel Sonnar that does have 40.5 mm threads and a chrome collapsible 5 cm f/2.8 Tessar that does have 25.5 mm threads. I'm not certain that even if I found a 27 mm push on UV filter that it would fit the black and nickel Tessar that I have since the 25.5 filter is too big to fit it and I think that the outside diameter of the threaded portion seems a bit smaller than the 2.8 lens. I did find a 24 mm UV filter on eBay so I may give that a try.
You have a very nice looking Tessar there. Like you I would have expected it to have 27mm slip on and 25.5mm screw filter sizes. They were the standard ones for the Zeiss Jena 5cm Tessars. Is it possible that your filter 25.5mm thread has paint or dirt or a small ding in it. Look next to the spanner slot above the word "Carl".
The threads look clean to me and don't appear damaged in any way. I take your point about UV filters not being in common use when this lens was made, but as I'm not sure how soft the glass was for this front element I'm inclined to be cautious. There are too many Leica Summars and collapsible Zeiss Sonnar around that produce soft images due to damaged front elements.
Thanks for the teflon tape suggestion--when I get my 24 mm filter I will give that a try if it doesn't fit.
