58mm - Hoya 58mm CP HD Filter
They also boast specialised coatings that are not just waterproof and repel water spots but also hardened to be scratch and stain resistant. Of course these coatings are from HOYA so they greatly reduce reflections off the surface of the glass allowing you to capture more light in your photos.
The polarising film is the same as that used in the latest high definition LCD TV screens. It has 25% higher light transmission than standard polarizing film used in current photographic filters.