I've always used a 43mm filter. One thing to watch out for: On my vintage 35/1.8, if the filter is too snug a fit, then when I go to remove it, the entire front lens group unscrews from the lens housing, exposing the aperture blades. From there on it's easy to remove the filter and reassemble the lens, but it can slow you down and be a bit disconcerting when it happens. There does not seem to be a set-screw to hold the lens group in place, just friction.