for slot in type filter..................... it usually comes this way, which i mentioned earlier.
you have:
1) FILTER HOLDER RING, which screws into lens, for different lens diameter, you will need different filter holder ring.
2) FILTER HOLDER slides onto the ring
3) FILTER slides into filter holder
so yes, you need specific diameter filter holder ring, but these probably cost less than buying individual screw in filters, e.g. nd4, nd8, etc............
Ok, I got it now.. thanks so much!