Sigma WR Ceramic Protector (available from 67mm to 105mm) Interchangeable lenses are the most important element of photographic expression, and to the photographer they are a valuable personal asset. To help photographers continue to use for many years the lens systems in which they have invested their time, knowledge, and budgets,SIGMA has developed original lens filters that meet the company's high standards of excellence. In developing its new protective lens filter, SIGMA has leveraged an unprecedented cooperative relationship with a glass manufacturer, resulting in a completely new type of glass called Clear Glass Ceramic. Featuring this advanced material, the SIGMA WR CERAMIC PROTECTOR is a highly reliable filter with outstanding protective capability. With many applications in aerospace and other industries, glass ceramic is an extremely tough type of crystallized glass that serves as the foundation for Clear Glass Ceramic. While featuring the high transmittance required of optical devices, this advanced new material combines greater hardness than chemically strengthened glass and greater flexibility than sapphire crystal glass. These qualities make Clear Glass Ceramic the ideal material for protective lens filters. Highly shock-resistant Clear Glass Ceramic In the chemically strengthened glass used in conventional high-strength protective filters, only the surface is strengthened through chemical processes. In contrast