The 18 multi-layer coatings are a key feature that significantly improves image quality. These coatings work by: Reducing lens flare and ghosting: This keeps your photos clear and sharp, even when shooting towards bright light sources. Improving light transmission: This ensures that colors are more accurate and vibrant, without any unwanted color cast. Making the filter water and scratch-resistant: The Nano-Klear coating makes the surface slick, so water and oil droplets bead up and are easy to wipe off. It also provides a durable layer of protection against minor scratches. 

The "HD Ultra-thin" design refers to the filter's slim frame, which is only 3.3mm for the CPL and 3.2mm for the MCUV. This is crucial for two reasons. First, the ultra-thin design helps to prevent vignetting (darkening at the corners of the image), especially when using wide-angle lenses. Second, it ensures the filter doesn't interfere with the lens cap, making it more convenient to leave the filter on your lens for quick shots.