B+W 77mm XS-Pro HTC Kaesemann Circular Polarizer
German-made B+W XS-Pro HTX Kaesemann Nano circular polarizers are the industry standards when it comes to performance, optical quality, and being built to last.
Kaesemann ("encased") polarizers are sealed against moisture, which is important if you take your kit into humid environments, frigidly cold climates, or anywhere there are dramatic changes in temperature. The Kaesemann version also incorporates special polarizing foils designed to be efficient and extra transmissive, meaning they are designed to block only the bare minimum of light. Where some polarizers can cut up to 3-stops as they polarize, the B+W Kaesemann block just 1.5-stops, otherwise yielding the same polarizing effect; that's quite a technological feat!
Nano is B+W's marketing term for their proprietary process that adds an eighth layer of coating meant to increase the water beading effect making the filter that much easier to clean and keep clean. In fact, this is an upgrade to B+W's MRC process which doesn't include this last layer. Nano filters include every layer and benefit of B+W's traditional multi-coating (MRC) process, as well, so expect this filter to fight flare exceptionally well and be easy to clean and maintain.
This filter utilizes B+W's XS-Pro filter rings which are quality brass-made rings with an ultra low profile yet still retain front threads for filter stacking. Being made of brass they should bind to other filters less, resisting getting stuck together, and they have a very low thickness so they can be mounted on wide-angle lenses without the filter rings coming into view as you get out to the wider focal lengths.
B+W filters are made with industry-leading Schott glass, and the XS-Pro HTC Kaesemann Nano Circular Polarizer is no different. B+W has really shoe/horned every feature they could into this filter. The result is a circular polarizer that is the best of the best, every other manufacturer is playing catch up. In fact, B+W's circular polarizers might be the most counterfeited filters in the world, so be mindful to purchase them from reputable, authorized dealers.