I'd look for more modern, multicoated filters if possible - and as the glass in these older Leica lenses is very soft (and the coating fragile), I'd keep a filter on as much as possible to reduce the need to clean the glass directly. I use color correction filters frequently and shoot modern, hard-coated lenses without filters otherwise. My Summar always has a filter on it as I want to keep it unscratched.

Modern multicoated filter don't add much in the way of flare. I have a Hoya MC filter on a Summar which works nicely. The only headache is that it makes filters that fit over the lens more of a headache. For example, a FIKUS works with the Summar without a filter but vignettes with one. I still haven't found a hood solution I really like. I'll probably try a DIY solution at some point.

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And plenty of the non-Leica glass is excellent. The Voigtlander lenses are generally safer purchases (less variability due to age than 50+ year old glass). The Yashinon 5cm 1.8 is a favorite of mine but the Canon LTM lenses are very close. The VC 35mm 2.5 LTM is an excellent 35mm option and a bargain compared to almost anything else out there.