Are there low-profile filters thin enough to leave on a Voigtländer Bessa III when folded?

A definite compatible filter isn’t identified in the replies, but the most plausible option mentioned is a low-profile/slim filter, such as Hoya’s slim designs or similar low-profile filters from other brands. These are typically made to reduce vignetting on wide-angle lenses and are thinner than standard filters.

That said, the community response also cautions against leaving a filter on a folding Bessa III when closing the camera. Clearance may be extremely tight, and there’s a real risk of interference or damage if the filter is even slightly too thick. A polariser is especially likely to be problematic because they are usually thicker than simple color filters.

So the practical takeaway is: look at slim/low-profile filters, but don’t assume they can safely stay on when folded unless you can test the exact filter on the camera very carefully.