Quality begins in front of the lens, and as is generally known, that is where the filter is. A filter must not restrict the performance of the photographic lens.
An important criteria for a perfect B+W filter is a high-quality coating, also known as an antireflection coating. With a B+W filter, such layers are evaporated by means of thin-layer technique, a quite complex coating process. The coating material consists of special substrates that are applied in one or more layers depending on the coating system.