38mm. I'm not sure if it's some wierd thread or not. The filter is part of the optical design and must be used so the lens comes with four filters.
1) A cooling filter for use with incadenscent lights. Not sure how blue the filter is.
2) green. This is supposed to be the standard Soviet YG but the seller called it green.
3) Orange. I wonder if this is really a dark yellow.
4) A clear neutral filter. This is the one I'd like to fit gel filters to.