I recently got myself Voigtlander Super Wide-Heliar 15mm/4.5 lens, which I enjoy shooting a lot. It has built-in lens hood with filter threads and manual says it's fitted with 52mm filter threads.
I want to use it with ND filter and when I got to the store I wasn't able to screw in a single filter because filter body seem to be too "wide" and diameter of the hood too narrow. I've also tried step-down rings and I had exactly the same problem, as rings were hitting the hood.
Should I looks for some kind of special filters to fit this lens?
Lens photo for the reference (you can see threads inside the hood):